Optical pickup apparatus and optical recording medium drive employing the same

ABSTRACT

A grating surface of a diffraction grating diffracts a laser beam emitted from a semiconductor laser device in ±1st order directions. The grating surface is formed in a rectangular or elliptic shape in such dimensions that a light spot formed on an objective lens by ±1st order diffracted beams is located in an aperture of the objective lens and not displaced from the aperture even if the objective lens is horizontally moved in a tracking operation. A grating surface of another diffraction grating has a width smaller than the width of an overlap region of a light spot on the diffraction grating corresponding to a part of a beam, diffracted in the +1st direction, entering an objective lens and a light spot on the diffraction grating corresponding to a part of a beam, diffracted in the −1st direction, entering the objective lens.

BACKGROUND OF THE INVENTION

1. 1. Field of the Invention

2. The present invention relates to an optical pickup apparatus and anoptical recording medium drive employing the same.

3. 2. Description of the Prior Art

4. An optical pickup apparatus employed for an optical recording mediumdrive such as an optical disk drive is adapted to record or readinformation in or from an optical recording medium such as an opticaldisk or detect a servo signal with a laser beam.

5.FIG. 20 schematically illustrates a conventional optical pickupapparatus disclosed in Japanese Patent Laying-Open Gazette No. 3-76035(1991). This optical pickup apparatus performs tracking servo control bythe three-beam method.

6. Referring to FIG. 20, symbols X, Y and Z denote the radial directionof an optical disk 1, the track direction of the optical disk 1, and adirection perpendicular to the disk plane of the optical disk 1respectively.

7. A semiconductor laser device 102 emits a laser beam B in thedirection Z. The beam B emitted from the semiconductor laser device 102enters a diffraction grating 103. FIG. 21 is a plan view of thediffraction grating 103. The diffraction grating 103 has a gratingsurface 103 a formed by unevenness of regular pitches. The gratingsurface 103 a divides the incident laser beam B into three beams, i.e.,a 0th order diffracted beam (main beam), a +1st order diffracted beam(subbeam) and a −1st order diffracted beam (subbeam), and transmits thesame through a transmission-type holographic optical element 104.

8. Referring to FIG. 20, an objective lens 105 condenses the three beamstransmitted through the transmission-type holographic optical element104 on the optical disk 1. FIG. 22 is a model diagram showing thecondensed states on the recording plane of the optical disk 1. As shownin FIG. 22, the 0th order diffracted beam is condensed on a tracksurface TR of the recording plane as a main spot M0, and the ±1st orderdiffracted beams are condensed on both sides of the main spot M0 assubspots S1 and S2 respectively.

9. The transmission-type holographic optical element 104 diffracts threereturned beams (reflected beams) from the main spot M0 and the subspotsS1 and S2 in a plane substantially including the directions X and Z, sothat a photodetector 106 detects these returned beams.

10.FIG. 23 is a typical plan view showing an exemplary photodetector106. This photodetector 106 includes a photodetection part 106 aprovided on the central portion for performing focus servo control withthe astigmatism method and photodetection parts 106 b and 106 c providedon both sides of the photodetection part 106 a for performing trackingservo control with the three-beam method. The returned beamcorresponding to the main spot M0 enters the central portion of thephotodetection part 106 a while the returned beams corresponding to thesubspots S1 and S2 enter the photodetection parts 106 b and 106 crespectively.

11. The aforementioned optical pickup apparatus performs trackingcontrol in the following manner: As shown in FIG. 22, the track surfaceTR recording information is different in light reflectance from anon-track surface. When the photodetection parts 106 b and 106 c detectthe returned beams from the subspots S1 and S2, the returned beams fromthe two subspots S1 and S2 entering the two photodetection parts 106 band 106 c are equal in light intensity to each other if the main spot M0excellently tracks the track surface TR to be reproduced. If the mainspot M0 deviates to either side of the track surface TR, on the otherhand, the photodetection part 106 a or 106 b relatively largely detectsthe light intensity of the returned beam from one of the subspots S1 andS2. With output signals E and F from the photodetection parts 106 b and106 c, therefore, the following tracking error signal TE is obtained:

12.  TE=E−F

13.

14. The optical pickup apparatus performs excellent tracking controlwhen the tracking error signal TE is zero, and detects deterioration ofthe tracking state as the value of the tracking error signal TEincreases.

15. When detecting the tracking error signal TE, the optical pickupapparatus moves the objective lens 105 in the radial direction (thedirection X), for correcting the condensed positions of the main spot M0and the subspots S1 and S2 on the track surface TR of the optical disk1.

16.FIG. 24A is a typical sectional view showing the condensed states ofdiffracted beams B1 and B2 diffracted by the diffraction grating 103,and FIG. 24B shows typical plan views of the objective lens 105. Asshown in FIG. 24A, the diffracted beam B1 diffracted by the diffractiongrating 103 in the +1st order direction passes through the objectivelens 105, to be condensed as the subspot S1. The diffracted beam B2diffracted in the −1st order direction passes through the objective lens105, to be condensed as the subspot S2.

17. Referring to FIG. 24B, the grating surface 103 a of the diffractiongrating 103 is formed to be larger than the laser beam B, as shown inFIG. 20. Therefore, the laser beam B incident on the grating surface 103a is diffracted over a region wider than an aperture 105 a of theobjective lens 105, to result in regions B1 a and B2 a not entering theaperture 105 a of the objective lens 105.

18. When the optical pickup apparatus performs a tracking operation inthis state and moves the objective lens 105 in the direction X (theradial direction of the optical disk 1), the incident states of thediffracted beams B1 and B2 on the objective lens 105 change from thoseon the left to those on the right in FIG. 24B. The ratios of thediffracted beams B1 and B2 entering the aperture 105 a of the objectivelens 105 reduce following movement of the objective lens 105. Therefore,the light quantities of the subspots S1 and S2 reduce on the recordingplane la of the optical disk 1, to result in reduction of the lightquantities of the returned beams from the subspots S1 and S2 enteringthe photodetection parts 106 b and 106 c. When the objective lens 105 ismoved during the tracking operation, therefore, the output of thetracking error signal TE disadvantageously reduces.

19.FIG. 25 is a model diagram for illustrating the diffracted state ofthe beam B diffracted by the diffraction grating 105. Referring to FIG.25, a light source 200 forms an emissive end of the semiconductor laserdevice 102, so that the laser beam B emitted from this light source 200is condensed on the recording plane 1 a of the optical disk 1 as the twosubspots S1 and S2. The transmission-type holographic optical element104 is omitted in FIG. 25.

20. The grating surface 103 a diffracts the laser beam B emitted fromthe light source 200 at least in the +1st order direction and the −1storder direction. In the laser beam B, the +1st order diffracted partialbeam of a partial beam BE1 passes through the objective lens 105, to becondensed as the subspot S1. The +1st order diffracted partial beam of apartial beam BE2 passes through a part beyond the objective lens 105,not to be condensed on the subspot S1.

21. On the other hand, the −1st order diffracted partial beam of apartial beam BE3 passes through the objective lens 105, to be condensedon the subspot S2. Further, the −1st order diffracted partial beam of apartial beam BE4 passes through a part beyond the objective lens 105,not to be condensed on the subspot S2.

22. When an optical axis LP passing through the peak of the lightintensity distribution of the laser beam B aligns with a central axis Z0passing through the center of the objective lens 105, the lightquantities of the partial beams BE1 and BE3 condensed on the subspots S1and S2 respectively are equal to each other. Therefore, the correcttracking state can be detected by detecting the difference between thelight quantities of the returned beams from the two subspots S1 and S2.

23. However, the optical axis LP of the laser beam B may deviate fromthe central axis Z0 of the objective lens 105 due to a locational errorof the semiconductor laser device 102 or the emission property of thelaser beam B. When the optical axis LP deviates from the central axisZ0, the partial beams BE1 and BE3 are condensed on the two subspots S1and S2 in non-uniform light quantities.

24.FIGS. 26A and 26B illustrate light intensity distribution states ofthe laser beam B in a section taken along the line Q-Q in FIG. 25. InFIGS. 26A and 26B, a symbol 2R denotes the diameter of the partial beamincidenting into the objective lens 105 within the +1st and the −1storder diffracted beams. The optical axis LP aligns with the central axisZ0 in FIG. 26A, while the former deviates from the latter in FIG. 26B.FIG. 26A shows the light quantities corresponding to the partial beamsBE1 and BE2 in regions (E1+E2) and (E3) respectively. Further, the lightquantities corresponding to the partial beams BE3 and BE4 are shown inregions (E1+E3) and (E2) respectively.

25. As shown in FIG. 26A, the light quantity (the region (E1+E2)) of thepartial beam BE1 condensed on the subspot S1 is equal to the lightquantity (the region (E1+E3)) of the partial beam BE3 condensed on thesubspot S2 when the optical axis LP aligns with the central axis Z0.

26. When the optical axis LP deviates from the central axis Z0, on theother hand, the light quantities of the partial beams BE1 and BE3condensed on the subspots S1 and S2, which are shown in regions (E1+E20)and (E1+E30) respectively, differ from each other. Thus, the trackingerror signal TES based on the returned beams from the two subspots S1and S2 is so offset that it is difficult to detect the correct trackingstate.

27.FIG. 27 schematically illustrates another conventional optical pickupapparatus. This optical pickup apparatus is adapted to perform trackingservo control and focus servo control with the three-beam method and theastigmatism method respectively.

28. Referring to FIG. 27, a laser beam 112 emitted from a semiconductorlaser device 121 passes through a transmission-type diffraction grating123 to be divided into three beams, i.e., a 0th order diffracted beam(main beam) and ±1st order diffracted beams (subbeams) and transmittedthrough a transmission-type holographic optical element 124.

29. An objective lens 116 condenses the three beams transmitted throughthe transmission-type holographic optical element 124 on an optical disk1 as a main spot M0 and subspots S1 and S2 located on both sidesthereof. An actuator 140 supports the objective lens 116 to be movablein the radial direction (the X-axis direction) of the optical disk 1 fora tracking operation and to be movable in the Y-axis direction for afocus operation.

30.FIG. 28 illustrates the main spot M0 and the subspots S1 and S2formed on the optical disk 1. As shown in FIG. 28, the optical system ofthe optical pickup apparatus is so adjusted that the main spot M0 scansa track TR to be reproduced and the subspots S1 and S2 scan both sidesof the main spot M0 slightly over the track TR.

31. The transmission-type holographic optical element 124 diffractsthree returned beams (reflected beams) from the optical disk 1, so thata signal detection photodiode 133 detects the same.

32.FIG. 29 is a typical plan view showing an exemplary signal detectionphotodiode 133. This signal detection photodiode 133 includesphotodetection parts 150 a to 150 d provided on the central portion forperforming focus servo control with the astigmatism method andphotodetection parts 150 e and 150 f provided on both sides of thephotodetection parts 150 a to 150 d for performing tracking servocontrol with the three-beam method. The returned beam (main beam)corresponding to the main spot M0 enters the central portion of thephotodetection parts 150 a to 150 d, while returned beams (subspots) 112a and 112 b corresponding to the subspots S1 and S2 enter thephotodetection parts 150 e and 150 f respectively.

33. On the basis of detection signals E and F from the photodetectionparts 150 e and 150 f of the signal detection photodiode 133 receivingthe returned beams (subbeams) 112 a and 112 b, the optical pickupapparatus performs the tracking operation in the following manner:

34.FIG. 30 is a circuit diagram showing respective parts of an opticaldisk drive comprising the optical pickup apparatus 100 performing thetracking operation. Referring to FIG. 30, the photodetection parts 150 eand 150 f of the signal detection photodiode 133 of the optical pickupapparatus 100 output the detection signals E and F to an E-F processingpart 155 provided on a driving circuit part 154 of the optical diskdrive. With the detection signals E and F received from thephotodetection parts 150 e and 150 f, the E-F processing part 155obtains the following tracking error signal TE:

TE=E−F

35. The tracking error signal TE is inputted in an operational amplifier158 of a servo circuit 157 through a low-pass filter 156, amplified andthereafter supplied to a tracking coil 142 of the actuator 140 of theoptical pickup apparatus 100.

36. As shown in FIG. 27, the actuator 140 supports the objective lens116 to be movable in the radial direction (the X-axis direction) of theoptical disk 1. The actuator 140 comprises a holder 141 for holding theobjective lens 116, the tracking coil 142 connected to the holder 141 tobe movable in the radial direction, and a permanent magnet 144separating from the tracking coil 142. When a driving voltage is appliedto the tracking coil 142, the actuator 140 moves the objective lens 116in the X-axis direction by electromagnetic force caused between thetracking coil 142 and the permanent magnet 144.

37. When the main spot M0 formed on the optical disk 1 effectivelytracks the track TR to be reproduced in FIG. 28, the returned beams 112a and 112 b from the two subspots S1 and S2 enter the photodetectionparts 150 e and 150 f in equal light intensity. Therefore, the trackingerror signal TE outputted from the E-F processing part 155 is zero andno driving voltage is applied to the tracking coil 142 of the actuator140. Thus, the objective lens 116 maintains its state.

38. When the main spot M0 deviates to either side of the track TR to bereproduced, on the other hand, the light intensity of the returned beam112 a or 112 b from the subspot S1 or S2 increases. Thus, the detectionsignals E and F from the photodetection parts 150 e and 150 f differfrom each other. Therefore, the E-F processing part 155 outputs thetracking error signal TE, which in turn is amplified by the operationalamplifier 158 of the servo circuit 157 so that a driving voltage isapplied to the tracking coil 142 and the actuator 140 radially moves theobjective lens 116 for correcting the position of the main spot M1.

39. In recent years, miniaturization of such an optical pickup apparatus100 is strongly desired, and the respective elements thereof areminiaturized with reduction of the diameter of the objective lens 116.In an assembling step for the optical pickup apparatus 100, therefore,it is difficult to correctly align the objective lens 116 with theoptical path of the laser beam 112.

40.FIG. 31 is a typical plan view showing an incident state of the laserbeam 112 on the objective lens 116. In the optical pickup apparatus 100,the semiconductor laser device 121, the diffraction grating 123 and thetransmission-type holographic optical element 124 are integrated into aunit independently of the objective lens 116, and these units areassembled with each other in alignment. In assembling, therefore, theoptical axis of the objective lens 116 may deviate from those of the twosubbeams 112 a and 112 b of the laser beam 112 by d along the radialdirection (the X-axis direction) of the optical disk 1, as shown in FIG.31.

41. Such deviation d in the mounting position of the objective lens 116results in the following disadvantage: The optical disk drive moves theobjective lens 116 by a constant distance in the radial direction of theoptical disk 1 in order to search the program for a tune recorded in theoptical disk 1, for example. If the optical axis of the objective lens116 deviates from those of the subbeams 112 a and 112 b of the laserbeam 112 by d in assembling as shown in FIG. 31, however, the subbeams112 a and 112 b pass through the objective lens 116 in different lightquantities following movement of the objective lens 116 for the programsearch, in response to the direction of movement. The light quantitiesof the subbeams 112 a and 112 b passing through the objective lens 116extremely reduce following movement of the objective lens 116 in onedirection, and hence the output of the tracking error signal TE based onthe subbeams 112 a and 112 b passing through the objective lens 116reduces to hinder a effective tracking operation.

42.FIG. 32 illustrates changes of the tracking error signal TE followingmovement of the objective lens 116. Referring to FIG. 32, the horizontalaxis shows the direction and the distance of movement of the objectivelens 116, and the vertical axis shows the tracking error signal TE. Whenthe center of the objective lens 116 aligns with the optical axis of thelaser beam 112 in the radial direction of the optical disk 1,symmetrical distribution TE0 of the tracking error signal TE is obtainedfollowing movement of the objective lens 116, as shown by a dotted linein FIG. 32. When the center of the objective lens 116 deviates from theoptical axis of the laser beam 112, on the other hand, asymmetricaldistribution TE1 of the tracking error signal TE is obtained dependingon the direction of movement of the objective lens 116, as shown by asolid line. The tracking error signal TE reduces below an output value Anecessary for tracking on a position of movement of the objective lens116, to hinder correct program search.

43. In general, therefore, an offset circuit 159 is provided on oneinput side of the operational amplifier 158 of the servo circuit 157, inorder to correct the deviation of the objective lens 116 from theoptical axis of the laser beam 112. In the optical pickup apparatus 100built into the optical disk drive, the offset circuit 159 corrects thedeviation of the objective lens 116 as follows:

44. The offset circuit 159 moves the objective lens 116 along the radialdirection toward the center and the outer periphery respectively byprescribed distances of 400 μm, for example, and detects the voltages ofthe tracking error signal TE. If the center of the objective lens 116deviates from the optical axis of the laser beam 112, the tracking errorsignal TE1 exhibits different voltages following movement of theobjective lens 116 toward the center and the outer periphery, as shownin FIG. 32. Therefore, the movement origin position (the position of theobjective lens 116 performing no tracking operation) is moved forequalizing the voltages of the tracking error signal TE in movement ofthe objective lens 116 toward the center and the outer periphery.

45. The resistance value of a variable resistor 160 of the offsetcircuit 159 is adjusted and a driving voltage is applied to the trackingcoil 142 for moving the movement origin position of the objective lens116 in the radial direction of the optical disk 1. Further, theobjective lens 116 is moved from the movement origin position along theradial direction of the optical disk 1 toward the center and the outerperiphery by prescribed distances respectively, for detecting thecurrent values of the tracking error signal TE. Adjustment of thevariable resistor 160 of the offset circuit 159 is ended when thedetected values of the tracking error signal TE are equal to each otherin movement toward the center and the outer periphery. Thus, thedeviation of the objective lens 116 from the optical axis of the laserbeam 112 in the radial direction of the optical disk 1 can be corrected.

46. However, the optical pickup apparatus 100 may be independentlymanufactured and put on the market by a manufacturer different from thatfor the optical disk drive employing the same. In this case, therefore,the manufacturer for the optical disk drive or the like must adjust thedeviation of the objective lens 116 of the optical pickup apparatus 100with complicated assembling and adjusting operations.

SUMMARY OF THE INVENTION

47. An object of the present invention is to provide an optical pickupapparatus causing no output reduction of a tracking signal followingmovement of an objective lens in a tracking operation and an opticalrecording medium drive employing the same.

48. Another object of the present invention is to provide an opticalpickup apparatus capable of suppressing offset of a tracking errorsignal resulting from optical axis deviation of a beam emitted from alight source and an optical recording medium drive employing the same.

49. Still another object of the present invention is to provide anoptical pickup apparatus capable of adjusting the position of anobjective lens with respect to the optical axis of a laser beam inmanufacturing, an optical recording medium drive comprising the same,and a method of adjusting an optical pickup apparatus.

50. The optical pickup apparatus according to the present inventioncomprises a light source for emitting a beam, a first diffractionelement having a diffraction surface for diffracting the beam emittedfrom the light source at least in first and second directions, and anobjective lens for irradiating an optical recording medium with beamsdiffracted by the first diffraction element in the first and seconddirections respectively. The objective lens is provided to be movablealong the radial direction of the optical recording medium for atracking operation, and the diffraction surface of the first diffractionelement is formed in dimensions for locating a light spot formed on theobjective lens by the diffracted beams diffracted by the diffractionsurface in the first and second directions respectively in an apertureof the objective lens following movement of the objective lens for thetracking operation.

51. Also when the objective lens is moved for the tracking operation,all diffracted beams pass through the objective lens, to be condensed onthe optical recording medium. Therefore, the light quantities of thediffracted beams condensed on the optical recording medium remainunchanged regardless of movement of the objective lens. Thus, it ispossible to prevent the output of a tracking error signal fromfluctuation resulting from change of the light quantities of thediffracted beams on the optical recording medium resulting from to thetracking operation.

52. In particular, the diffraction surface of the first diffractionelement is preferably formed in a rectangular shape smaller than a lightspot formed on the first diffraction element by the beam emitted fromthe light source in dimensions for locating a rectangular light spotformed on the objective lens by the diffracted beams diffracted by thediffraction surface in the first and second directions respectivelyfollowing movement of the objective lens in the aperture of theobjective lens.

53. In this case, the rectangular light spot of the diffracted beamsenters the aperture of the objective lens regardless of the movement ofthe objective lens. Therefore, the light quantities of the diffractedbeams condensed on the optical recording medium are maintained constantalso when the objective lens is moved, whereby the tracking error signalbased on the diffracted beams can be prevented from fluctuation.

54. Particularly assuming that R and Q represent the aperture radius andthe amount of movement of the objective lens respectively, L1 and L2represent effective distances between the light source and the center ofthe objective lens and between the diffraction surface and the lightsource respectively, S represents the distance between a first virtuallight source supposed to emit a straight beam equivalent to the beamdiffracted in the first direction toward the objective lens and thelight source or between a second virtual light source supposed to emit astraight beam equivalent to the beam diffracted in the second directiontoward the objective lens and the light source, and B1 represents alimit value for the rectangular light spot formed on the objective lensin a direction perpendicular to the direction of movement of theobjective lens, the width W1 of the diffraction surface of the firstdiffraction element in the direction perpendicular to the direction ofmovement of the objective lens is preferably set to satisfy:

W1<2×{{square root}{square root over (R²−Q²)}−S}×L2/L1+2S

55. and the width W2 of the diffraction surface in the direction ofmovement of the objective lens is preferably set to satisfy:

W 2≦{{square root}{square root over ((2R)²−(B1)²)}−2Q}×L2/L1

56. with the following limit value B1:

B 1=( W1−2S)×L1/L2+2S

57. The diffraction surface satisfying the above conditions introducesall diffracted beams diffracted in the first and second directions intothe aperture of the objective lens regardless of movement of theobjective lens. Thus, the light quantities of the diffracted beams areprevented from variation with movement of the objective lens.

58. In particular, the optical pickup apparatus provided with the firstdiffraction element having the diffraction surface formed in arectangular shape may further comprise a second diffraction element fortransmitting the beams diffracted by the first diffraction element inthe first and second directions respectively and guiding the same to theobjective lens while diffracting returned beams from the opticalrecording medium, and a photodetector for receiving the returned beamsdiffracted by the second diffraction element.

59. In this case, the second diffraction element diffracts the returnedbeams, so that the returned beams from the optical recording medium canbe guided to the photodetector, which in turn detects the tracking errorsignal.

60. The diffraction surface of the first diffraction element ispreferably formed in an elliptic or circular shape smaller than thelight spot formed on the first diffraction element by the beam emittedfrom the light source in dimensions for locating an elliptic light spotformed on the objective lens by the diffracted beams diffracted by thediffraction surface in the first and second directions respectively inthe aperture of the objective lens following movement of the objectivelens.

61. In this case, all elliptic or circular diffracted beams can beintroduced into the aperture of the objective lens regardless ofmovement of the objective lens in the tracking operation.

62. Particularly assuming that the elliptic diffraction surface of thefirst diffraction element has its major axis in the directionperpendicular to the direction of movement of the objective lens, R andQ represent the aperture radius and the amount of movement of theobjective lens respectively, L1 and L2 represent effective distancesbetween the light source and the center of the objective lens andbetween the diffraction surface and the light source respectively, Srepresents the distance between a first virtual light source supposed toemit a straight beam equivalent to the beam diffracted in the firstdirection toward the objective lens and the light source or between asecond virtual light source supposed to emit a straight beam equivalentto the beam diffracted in the second direction toward the objective lensand the light source, b represents a limit value for the radius of theelliptic light spot formed on the objective lens in the directionperpendicular to the direction of movement of the objective lens and WBrepresents the width of the elliptic diffraction surface in thedirection perpendicular to the direction of movement of the objectivelens, the width WA of the elliptic diffraction surface in the directionof movement of the objective lens is preferably set to satisfy:

WA≦2×{{square root}{square root over (b² Q⁻² /(b²−R²)+b²)}}× L2/L1

63. where b=(WB−2S)×L1/L2+2S

64. Further, the width WB of the elliptic diffraction surface of thefirst diffraction element in the direction perpendicular to thedirection of movement of the objective lens is preferably set tosatisfy:

2×[L2/L1×{{square root}{square root over (R×(R−Q))}−S}+S]≦WB<2×[L2/L1×{{square root}{square root over (R²−Q²)}− S}+S]

65. Assuming that R and Q represent the aperture radius and the amountof movement of the objective lens respectively and L1 and L2 representeffective distances between the light source and the center of theobjective lens and between the diffraction surface and the light sourcerespectively, in addition, the width WA of the elliptic diffractionsurface in the direction of movement of the objective lens is preferablyset to satisfy:

WA≦2×(R−Q)×L2/L1

66. Assuming that S represents the distance between the first virtuallight source supposed to emit a straight beam equivalent to the beamdiffracted in the first direction toward the objective lens and thelight source or between the second virtual light source supposed to emita straight beam equivalent to the beam diffracted in the seconddirection toward the objective lens and the light source, further, thewidth WB of the elliptic diffraction surface of the first diffractionelement in the direction perpendicular to the direction of movement ofthe objective lens is preferably set to satisfy:

WB<2×[L2/L1×{{square root}{square root over (R×(R−Q))}− S}+S]

67. In particular, the optical pickup apparatus having the firstdiffraction element having the elliptically formed diffraction surfacemay further comprise a second diffraction element for transmitting thebeams diffracted by the first diffraction element in the first andsecond directions respectively, guiding the same to the objective lensand diffracting the returned beams from the optical recording medium,and a photodetector for receiving the returned beams diffracted by thesecond diffraction element.

68. In this case, the second diffraction element diffracts the returnedbeams from the optical recording medium for guiding the same to thephotodetector, so that the tracking error signal by the photodetectorcan be detected.

69. An optical pickup apparatus according to another aspect of thepresent invention comprises a light source for emitting a beam, a firstdiffraction element having a diffraction surface for diffracting thebeam emitted from the light source at least in first and seconddirections, and an objective lens for irradiating an optical recordingmedium with beams diffracted by the first diffraction element in thefirst and second directions respectively. The width of the firstdiffraction element in a plane including the optical axis of the beamemitted from the light source and axes of the beams diffracted in thefirst and second directions respectively is set to be smaller than thatof a region including a first light spot and a second light spot. Thefirst light spot is a light spot on the first diffraction elementcorresponding to a part of the beam, diffracted by the first diffractionelement in the first direction, entering the objective lens in the beamemitted from the light source. The second light spot is a light spot onthe first diffraction element corresponding to a part of the beam,diffracted by the first diffraction element in the second direction,entering the objective lens in the beam emitted from the light source.

70. The beam emitted from the light source includes the beam diffractedby the diffraction surface of the first diffraction element only in thefirst direction to enter the objective lens, a beam diffracted only inthe second direction to enter the objective lens, and a beam diffractedin the first and second directions to enter the objective lens. Thewidth of the diffraction surface of the first diffraction element isrendered smaller than that of the region including the first and secondlight spots formed on the first diffraction element, whereby the partcorresponding to the beam diffracted only in the first direction toenter the objective lens and that corresponding to the beam diffractedonly in the second direction to enter the objective lens can be reducedin a region incident on the diffraction surface. Thus, it is possible toinhibit the light quantities of the beams diffracted in the first andsecond directions from non-uniformity resulting from optical axisdeviation of the optical axis of the beam emitted from the light source,thereby suppressing non-uniform output of a tracking error detectionsignal utilizing the beams diffracted in the first and second directionsrespectively following optical axis deviation.

71. In particular, the width of the diffraction surface of the firstdiffraction element in the aforementioned plane is preferably set to besmaller than that of an overlap region of the first and second lightspots on the first diffraction element.

72. In this case, the light quantities of the diffracted beamsdiffracted in the first and second directions respectively to enter theobjective lens are equally changed even if the optical axis of the beamemitted from the light source deviates from a prescribed direction.Thus, the tracking error signal based on the diffracted beams in thefirst and second directions is reliably prevented from offset.

73. In particular, the first and second directions for diffracting thebeams by the diffraction surface of the first diffraction element arepreferably +1st and −1st order directions respectively.

74. Assuming that R represents the aperture radius of the objectivelens, L1 and L2 represent effective distances between the light sourceand the center of the objective lens and between the diffraction surfaceand the light source respectively and S represents the distance betweena first virtual light source supposed to emit a straight beam equivalentto the beam diffracted in the first direction toward the objective lensand the light source or between a second virtual light source supposedto emit a straight beam equivalent to the beam diffracted in the seconddirection toward the objective lens and the light source, the width W ofthe diffraction surface of the first diffraction element is preferablyset to satisfy the following relation:

W≦2×{(R+S)×L2/L1−S}

75. When the width W of the diffraction surface of the first diffractionelement is set to satisfy the above relation, the beams of the beamemitted from the light source diffracted in the first and seconddirections, based on a common beam part, enter the objective lens. Evenif the optical axis of the beam emitted from the light source deviates,therefore, the light quantities of the diffracted beams diffracted inthe first and second directions are equally changed. Thus, it ispossible to prevent offset of the tracking error signal based on thebeams diffracted in the first and second directions.

76. Assuming that X1 represents the physical distance between the lightsource and the center of the objective lens and d and n represent thethickness and the refractive index of the first diffraction elementrespectively, the effective distance L is defined as follows:

L1=X1−(n−1)×d/n

77. Assuming that X2 represents the physical distance between the lightsource and the diffraction surface and d and n represent the thicknessand the refractive index of the first diffraction element respectively,the effective distance L2 is defined as follows:

L2=X2−(n−1)×d/n

78. An optical pickup apparatus according to still another aspect of thepresent invention comprises a light source emitting a beam, a firstdiffraction element for diffracting the beam emitted from the lightsource at least in first and second directions, and an objective lensfor irradiating an optical recording medium with beams diffracted by thefirst diffraction element in the first and second directionsrespectively. The objective lens is provided to be movable along theradial direction of the optical recording medium for a trackingoperation, and the diffraction surface of the first diffraction elementis so formed that the width in a plane including the optical axis of thebeam emitted from the light source and axes of the beams diffracted inthe first and second directions respectively is smaller than that of aregion including a first light spot and a second light spot anddimensions are set for locating a light spot formed on the objectivelens by the beams diffracted by the diffraction surface in the first andsecond directions respectively in an aperture of the objective lensfollowing movement of the objective lens for the tracking operation. Thefirst light spot is a light spot on the first diffraction elementcorresponding to a part of the beam, diffracted by the first diffractionelement in the first direction, entering the objective lens in the beamemitted from the light source and the second light spot is a light spoton the first diffraction element corresponding to a part of the beam,diffracted by the first diffraction element in the second direction,entering the objective lens in the beam emitted from the light source.

79. In this case, the width of the diffraction surface of the firstdiffraction element is rendered smaller than that of the regionincluding the first and second light spots formed on the firstdiffraction element thereby reducing non-uniformity of the lightquantities of the diffracted beams in the first and second directionsresulting from optical axis deviation of the beam emitted from the lightsource, to be capable of suppressing non-uniform output of a trackingerror detection signal utilizing the diffracted beams diffracted in thefirst and second directions resulting from optical axis deviation.

80. Even if the objective lens is moved for the tracking operation, alldiffracted beams pass through the objective lens to be condensed on theoptical recording medium. Therefore, the light quantities of thediffracted beams condensed on the optical recording medium remainunchanged regardless of movement of the objective lens. Thus, it ispossible to prevent output fluctuation of the tracking error signalresulting from change of the light quantities of the diffracted beams onthe optical recording medium following the tracking operation.

81. In particular, the width of the diffraction surface of the firstdiffraction element in the aforementioned plane is preferably set to besmaller than that of an overlap region of the first and second lightspots formed on the first diffraction element.

82. In this case, the light quantities of the beams diffracted in thefirst and second directions to enter the objective lens are equallychanged even if the optical axis of the beam emitted from the lightsource deviates from a prescribed direction. Thus, it is possible toreliably prevent offset of the tracking error signal based on thediffracted beams in the first and second directions.

83. The optical pickup apparatus may further comprise a seconddiffraction element for guiding the beams diffracted by the firstdiffraction element in the first and second directions respectively andguiding the same to the objective lens while diffracting returned beamsfrom the optical recording medium and a photodetector for receiving thereturned beams diffracted by the second diffraction element.

84. In this case, the second diffraction element diffracts the returnedbeams from the optical recording medium for guiding the same to thephotodetector, so that the tracking error signal by the photodetectorcan be detected.

85. An optical pickup apparatus according to a further aspect of thepresent invention, which can detect a tracking state of a beam forreading information from an optical recording medium, comprises a lightsource for emitting the beam, a first diffraction element for dividingthe beam emitted from the light source into a plurality of beams fortracking state detection, an objective lens provided to be movable inthe radial direction of the optical recording medium for condensing theplurality of beams divided by the first diffraction element on theoptical recording medium, a photodetector having a plurality ofphotodetection parts for receiving a plurality of returned beams basedon the plurality of beams for tracking state detection respectively andoutputting a plurality of detection signals responsive to the receivedlight quantities, an adjusting circuit capable of changing the pluralityof detection signals outputted from the plurality of photoreceivingparts of the photodetector, and a lens driving part for radially movingthe objective lens in response to a prescribed signal based on theplurality of detection signals adjusted by the adjusting circuit.

86. The optical pickup apparatus according to this aspect of the presentinvention can change the plurality of detection signals for trackingstate detection by the adjusting circuit for moving the objective lensby the prescribed signal based on the changed detection signals. Thus,it is possible to correct deviation of the objective lens by adjustingthe adjusting circuit for changing the detection signals incorrespondence to the amount of deviation of the central portion of theobjective lens from the optical axis of the beam.

87. Further, the optical pickup apparatus itself is provided with theadjusting circuit, whereby the adjusting circuit can be adjusted in anassembling stage of the optical pickup apparatus. Thus, adjustment forcorrecting deviation of the objective lens can be omitted in anapparatus assembled with the optical pickup apparatus.

88. In particular, the adjusting circuit preferably includes a variableresistor for changing the plurality of detection signals outputted fromthe plurality of photoreceiving parts of the photodetector.

89. In this case, the resistance value of the variable resistor can beso adjusted as to readily change the prescribed signal supplied to thelens driving part, for correcting deviation of the objective lens.

90. In particular, the optical pickup apparatus may further comprise awiring part for extracting the signals from the plurality ofphotoreceiving parts of the photodetector, so that the variable resistoris arranged on the wiring part.

91. In this case, it is possible to readily change the prescribed signalsupplied to the lens driving part by adjusting the resistance value ofthe variable resistor arranged on the wiring part, for correctingdeviation of the objective lens.

92. In particular, the wiring part is preferably formed on a flexiblecircuit board. In this case, the degree of freedom in mounting of thewiring part in the optical pickup apparatus is improved due to theflexibility of the flexible circuit board, so that the optical pickupapparatus can be minimized.

93. In particular, the optical pickup apparatus preferably furthercomprises a plurality of amplifier parts provided in correspondence tothe plurality of photoreceiving parts in the photodetector foramplifying differences between the detection signals outputted from thecorresponding photoreceiving parts and a reference signal respectively,and the adjusting circuit preferably includes a variable resistor forchanging the reference signal supplied to at least one of the pluralityof amplifier parts.

94. In this case, it is possible to readily change the prescribed signalsupplied to the lens driving part by adjusting the variable resistor andchanging the reference signal, for correcting deviation of the objectivelens.

95. In particular, the photoreceiving parts and the plurality ofamplifier parts are formed on a single chip. In this case, the opticalpickup apparatus is suitable for miniaturization.

96. The optical recording medium drive according to the presentinvention, which is adapted to optically read information from anoptical recording medium, comprises a rotation driving part for rotatingthe optical recording medium, an optical pickup apparatus forirradiating the optical recording medium with a laser beam and receivinga returned beam from the optical recording medium, a pickup driving partfor moving the optical pickup apparatus in the radial direction of theoptical recording medium, and a signal processing part for processing anoutput signal from the optical pickup apparatus. Further, the opticalpickup apparatus comprises a light source for emitting the beam, adiffraction element having a diffraction surface for diffracting thebeam emitted from the light source at least in first and seconddirections, and an objective lens for irradiating the optical recordingmedium with beams diffracted by the diffraction element in the first andsecond directions respectively. The objective lens is provided to bemovable along the radial direction of the optical recording medium for atracking operation, and the diffraction surface of the diffractionelement is formed in dimensions for locating a light spot formed on theobjective lens by the beams diffracted by the diffraction surface in thefirst and second directions respectively in an aperture of the objectivelens following movement of the objective lens for the trackingoperation.

97. Thus, the light quantities of the diffracted beams on the opticalrecording medium remain unchanged in the tracking operation, and anoptical recording medium drive causing no output reduction of a trackingerror signal can be obtained.

98. An optical recording medium drive unit according to a further aspectof the present invention, which is adapted to optically read informationfrom an optical recording medium, comprises a rotation driving part forrotating the optical recording medium, an optical pickup apparatus forirradiating the optical recording medium with a laser beam and receivinga returned beam from the optical recording medium, a pickup driving partfor moving the optical pickup apparatus in the radial direction of theoptical recording medium, and a signal processing part for processing anoutput signal from the optical pickup apparatus. Further, the opticalpickup apparatus comprises a light source for emitting the beam, adiffraction element having a diffraction surface for diffracting thebeam emitted from the light source at least in first and seconddirections, and an objective lens for irradiating the optical recordingmedium with beams diffracted by the diffraction element in the first andsecond directions respectively. The width of the diffraction surface ofthe diffraction element in a plane including the optical axis of thebeam emitted from the light source and axes of the beams diffracted inthe first and second directions is set to be smaller than the width of aregion including a first light spot and a second light spot. The firstlight spot is a light spot on the diffraction element corresponding to apart of the beam, diffracted by the diffraction element in the firstdirection, entering the objective lens in the beam emitted from thelight source and the second light spot is a light spot on thediffraction element corresponding to a part of the beam, diffracted bythe diffraction element in the second direction, entering the objectivelens in the beam emitted from the light source.

99. In this case, offset of a tracking error signal is prevented even ifthe optical axis of the beam from the light source in the optical pickupapparatus deviates, whereby no offset adjustment is required and correcttracking control can be performed.

100. An optical recording medium drive according to a further aspect ofthe present invention, which is adapted to optically read informationfrom an optical recording medium, comprises a rotation driving part forrotating the optical recording medium, an optical pickup apparatus forirradiating the optical recording medium with a laser beam and receivinga returned beam from the optical recording medium, a pickup driving partfor moving the optical pickup apparatus in the radial direction of theoptical recording medium, and a signal processing part for processing anoutput signal from the optical pickup apparatus. Further, the opticalpickup apparatus comprises a light source for outputting the beam, adiffraction element for dividing the beam emitted from the light sourceinto a plurality of beams for tracking state detection, an objectivelens provided to be movable in the radial direction of the opticalrecording medium for condensing the plurality of beams divided by thediffraction element on the optical recording medium, a photodetectorhaving a plurality of photoreceiving parts for receiving a plurality ofreturned beams based on the plurality of beams for tracking statedetection condensed on the optical recording medium respectively foroutputting detection signals responsive to the received lightquantities, an adjusting circuit capable of changing the plurality ofdetection signals outputted from the plurality of photoreceiving partsof the photodetector, and a lens driving part for radially moving theobjective lens in response to a prescribed signal based on the pluralityof detection signals adjusted by the adjusting circuit.

101. The optical pickup apparatus adjusted to output a tracking signalfor correcting deviation of the objective lens is assembled into theoptical recording medium drive according to the present invention,whereby no adjustment of deviation of the objective lens of the opticalpickup apparatus is required after assembling and the optical recordingmedium drive is easy to assemble.

102. The method of adjusting an optical pickup apparatus according tothe present invention, which is adapted to correct deviation of acentral portion of an objective lens with respect to the optical axes ofa plurality of beams in the radial direction of an optical recordingmedium in the optical pickup apparatus comprising a light source foremitting a beam, a diffraction element for dividing the beam emittedfrom the light source into the plurality of beams for tracking statedetection, the objective lens for condensing the plurality of beamsdivided by the diffraction element on the optical recording medium, alens driving part for moving the objective lens in the radial directionof the optical recording medium, and a photodetector having a pluralityof photoreceiving parts for receiving a plurality of returned beamsbased on the plurality of beams for tracking state detection condensedon the optical recording medium respectively and outputting a pluralityof detection signals responsive to the received light quantities,provides the optical pickup apparatus with an adjusting circuit capableof changing the detection signals outputted from the plurality ofphotoreceiving parts, connects a driving circuit for generating adriving signal for moving the objective lens in the radial direction onthe basis of the detection signals outputted from the photodetectorthrough the adjusting circuit to the lens driving part of the opticalpickup apparatus, moves the objective lens in the radial direction bychanging the detection signals with the adjusting circuit and thereafterobserves change of the detection signals while radially moving theobjective lens by a prescribed distance, thereby correcting deviation ofthe central portion of the objective lens with respect to the opticalaxes of the plurality of beams in the radial direction of the opticalrecording medium.

103. The method of adjusting an optical pickup apparatus according tothe present invention connects the previously prepared driving circuitto the lens driving part of the optical pickup apparatus, changes theplurality of detection signals for tracking state detection with theadjusting circuit, and moves the objective lens by the prescribed signalbased on the changed detection signals. The method further reciprocatesthe objective lens in the radial direction by prescribed distances forinspecting the state of deviation of the objective lens, obtains thecurrent detection signals, and observes change of the detection signals.The adjusting circuit adjusts the detection signals to attain desiredvalues. Thus, it is possible to correct deviation of the central portionof the objective lens by adjusting the adjusting circuit to change thedetection signals in response to the amount of deviation of the mountingposition of the objective lens with respect to the optical axes of thebeams.

104. Further, the optical pickup apparatus itself is provided with theadjusting circuit, whereby the adjusting circuit can be adjusted in anassembling stage of the optical pickup apparatus. Thus, adjustment forcorrecting deviation of the central portion of the objective lens of theoptical pickup apparatus can be omitted in the apparatus assembled withthe optical pickup apparatus.

105. The foregoing and other objects, features, aspects and advantagesof the present invention will become more apparent from the followingdetailed description of the present invention when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

106.FIG. 1 is a schematic block diagram showing an optical pickupapparatus according to a first embodiment of the present invention;

107.FIG. 2A is a plan view of a diffraction grating, and FIG. 2B is aplan view showing incident states of diffracted beams, diffracted by thediffraction grating, on an objective lens;

108.FIG. 3 is a typical X-Z sectional view showing an optical diskirradiated with a laser beam;

109.FIG. 4 is a typical Y-Z sectional view showing the optical diskirradiated with the laser beam;

110.FIG. 5 is an explanatory diagram showing the relation betweeneffective and actual positions of a light source;

111.FIG. 6A is a plan view of another diffraction grating, and FIG. 6Bis a plan view showing an incident state of diffracted beams on theobjective lens;

112.FIG. 7A is a plan view of still another diffraction grating, andFIG. 7B is a plan view showing an incident state of diffracted beams onthe objective lens;

113.FIG. 8 is a schematic block diagram showing an optical pickupapparatus according to a second embodiment of the present invention;

114.FIG. 9 is a model diagram showing an optical disk irradiated with alaser beam;

115.FIG. 10 is a plan view showing an incident state of the laser beamon the diffraction grating;

116.FIG. 11 is an explanatory diagram showing the relation betweeneffective and actual positions of a light source;

117.FIG. 12 is a side sectional view showing the structure of an opticalpickup apparatus according to a third embodiment of the presentinvention;

118.FIG. 13 is an exploded perspective view of aprojecting/photoreceiving unit of the optical pickup apparatus shown inFIG. 12;

119.FIG. 14 is a plan view of a flexible circuit board of the opticalpickup apparatus shown in FIG. 12;

120.FIG. 15A is a circuit diagram of the flexible circuit board shown inFIG. 14, and FIG. 15B is a plan view of photoreceiving parts of a PDICmounted on the flexible circuit board;

121.FIG. 16 is a circuit diagram of respective parts of the opticalpickup apparatus shown in FIG. 12 for performing a tracking operation;

122.FIG. 17 is a circuit diagram showing a part around an adjustingcircuit of an optical pickup apparatus according to a modification ofthe third embodiment of the present invention;

123.FIG. 18 is a plan view showing another exemplary photoreceiving partof a signal detection PDIC;

124.FIG. 19 is a block diagram of an optical recording medium driveaccording to a fourth embodiment of the present invention;

125.FIG. 20 is a schematic block diagram of a conventional opticalpickup apparatus;

126.FIG. 21 is a plan view of a diffraction grating of the conventionaloptical pickup apparatus;

127.FIG. 22 is a model diagram showing condensed states on an opticaldisk;

128.FIG. 23 is a plan view showing the structure of a conventionalphotodetector;

129.FIG. 24A is a typical sectional view showing states of diffractedbeams, and FIG. 24B is a typical plan view showing the incident statesof the diffracted beams to an objective lens;

130.FIG. 25 is a schematic block diagram of the conventional opticalpickup apparatus:

131.FIGS. 26A and 26B are explanatory diagrams showing light intensitydistribution states of a laser beam;

132.FIG. 27 is a model diagram showing the structure of anotherconventional optical pickup apparatus;

133.FIG. 28 is a model plan view showing condensed states on an opticaldisk;

134.FIG. 29 is a plan view of a signal detection photodiode;

135.FIG. 30 is a circuit diagram of respective parts, performing atracking operation, of an optical disk drive comprising the conventionaloptical pickup device;

136.FIG. 31 is a typical plan view showing an incident state of a laserbeam on an objective lens; and

137.FIG. 32 illustrates change of a tracking error signal followingmovement of the objective lens.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

138. (1) First Embodiment

139. An optical pickup apparatus 100 shown in FIG. 1 is adapted toperform focus servo control with the astigmatism method and trackingservo control with the three-beam method.

140. Referring to FIG. 1, symbols X, Y and Z denote the radial directionof a reflection type optical disk 1 such as a CD (compact disk), thetrack direction of the optical disk 1 and a direction perpendicular to arecording plane 1 a of the optical disk 1 respectively.

141. The optical pickup apparatus 100 comprises aprojecting/photoreceiving unit 10 and an objective lens 5. Theprojecting/photoreceiving unit 10 is formed by a semiconductor laserdevice 2, a transmission-type diffraction grating 3, a transmission-typeholographic optical element 4 and a photodetector 6. A block 8 isprovided on a base 7, and a heat sink 9 is mounted on a side surface ofthe block 8. The semiconductor laser device 2 is mounted on a surface ofthe heat sink 9.

142. The transmission-type diffraction grating 3 is made of opticalglass or optical resin, and arranged in a holder 11 through a spacer 12.The transmission-type holographic optical element 4 is arranged in anopening on an upper surface of the holder 11.

143. The semiconductor laser device 2 emits a laser beam B in thedirection Z. The diffraction grating 3 divides the beam B emitted fromthe semiconductor laser device 2 into three beams, i.e., a 0th orderdiffracted beam (main beam), a +1st order diffracted beam (subbeam) anda −1st order diffracted beam (subbeam) in a plane, perpendicular to theplane of FIG. 1, substantially including the directions Y and Z, andtransmits the same through the transmission-type holographic opticalelement 4. FIG. 1 illustrates the three beams as a single beam.

144. The objective lens 5 is supported to be movable in the radialdirection (the direction X) of the optical disk 1 for tracking servocontrol, and to be movable in the vertical direction (the direction Z)for focus servo control. The objective lens 5 condenses the main beamand the two subbeams diffracted and transmitted through thetransmission-type holographic optical element 4 in the 0th and ±1storders respectively on the optical disk 1 as a main spot M0 and subspotsS1 and S2 positioned on both sides thereof respectively (see FIG. 22).

145. The transmission-type holographic optical element 4 diffracts threereturned beams (reflected beams) from the optical disk 1 in a planesubstantially including the directions X and Z in the 1st order andintroduces the same into the photodetector 6. At this time, thetransmission-type holographic optical element 4 supplies the threereturned beams from the optical disk 1 with astigmatism respectively.

146. The photodetector 6, which is similar in structure to thephotodetector 106 of the conventional optical pickup apparatus 100 shownin FIG. 23, outputs an information reproduction signal and a focussignal on the basis of the returned beam from the main spot M0 on theoptical disk 1 while outputting a tracking error signal TE on the basisof the returned beams from the subspots S1 and S2.

147. In the optical pickup apparatus 100 according to this embodiment,the shape and dimensions of a grating surface 3 a of the diffractiongrating 3 are so set that a light spot formed by the ±1st orderdiffracted beams entering the objective lens 5 is not displaced beyondits aperture 5 a following movement of the objective lens 5 in theradial direction (the direction X) of the optical disk 1 for a trackingoperation. A method of setting the grating surface 3 a is now described.

148. (1) Rectangular Grating Surface 3 a

149.FIG. 2A is a plan view of the diffraction grating 3, and FIG. 2B isa plan view showing incident states of the diffracted beams, diffractedby the diffraction grating 3 shown in FIG. 2A, on the objective lens 6.FIG. 3 is a typical X-Z sectional view showing the optical disk 1irradiated with the laser beam B, and FIG. 4 is a typical Y-Z sectionalview similar to FIG. 3.

150. Referring to FIG. 2A, the grating surface 3 a of the diffractiongrating 3 is formed in a rectangular shape having a vertical dimensionW1 in the direction (the Y-Z plane direction) for diffracting the laserbeam B and a transverse dimension W2 in the direction of movement (thedirection X) of the objective lens 5, with irregularities extending inthe direction X at regular pitches. The grating surface 3 a is formed tobe smaller than a light spot B0 formed on the grating surface 3 a by thelaser beam B emitted from the semiconductor laser device 2.

151. The diffracted beams diffracted by the grating surface 3 a in the±1st order directions form a rectangular light spot BS in the aperture 5a of the objective lens 5, as shown in FIG. 2B. Namely, the diffractedbeams transmitted through and diffracted by the rectangular gratingsurface 3 a having the vertical and transverse dimensions W1 and W2 formthe rectangular light spot BS having vertical and transverse dimensionsB1 and B2 in the aperture 5 a of the objective lens 5.

152. It is assumed here that the vertical dimension B1 of the light spotBS on the objective lens 5 has a given maximum dimension. The amount Qof movement of the objective lens 5 in the tracking operation ispreviously defined.

153. Therefore, the diffraction grating surface 3 a may be so set thatthe vertical dimension W1 is less than the maximum value of the verticaldimension B1 of the light spot BS and the transverse dimension W2 is atsuch a value that the light spot BS is not displaced beyond the aperture5 a of the objective lens 5 when relatively moved by Q in the directionX following movement of the objective lens 5.

154. Referring to FIG. 2B, the vertical dimension B1 of the light spotBS on the objective lens 5 is first given. The light spot BS is notdisplaced beyond the aperture 5 a of the objective lens 5 followingmovement of the objective lens 5 by Q in the direction X under thefollowing condition:

B2/2≦C−Q  (1)

155. In the above expression, the variable C is expressed as follows:

C={square root}{square root over (R²−(B1/2)²)}  (2)

156. Hence, the following relation is obtained for the transversedimension B2 of the light spot BS:

B2≦{square root}{square root over ((2R)² −B1²)}−2Q  (3)

157. Hence, the transverse dimension W2 of the grating surface 3 acorresponding to the transverse dimension B2 of the light spot BS on theobjective lens 5 is obtained as follows, on the basis of the geometricconditions of the optical system shown in FIG. 3:

W2=B2×L2/L1  (4)

158. From the expressions (3) and (4), the following relation isobtained for the transverse dimension W2 of the grating surface 3 a:

W2≦{{square root}{square root over ((2R)²−(B1)²)}−2Q}×L2/L1  (5)

159. where B1=(W1−2S)×L1/L2+2S

160. The vertical dimension W1 of the grating surface 3 a is obtained inthe following manner: B2>0 holds for the transverse dimension B2 of thelight spot BS in the expression (3), and hence:

{square root}{square root over ((2R)² −B1²)}−2Q>0  (6)

161. Hence,

B1<2×{square root}{square root over (R²Q²)}  (7)

162. This is the limit condition for the vertical dimension B1 of thelight spot BS on the objective lens 5 previously given as the maximumvalue.

163. As to the vertical dimension W1 of the grating surface 3 acorresponding to the vertical dimension B1 of the light spot BS formedon the objective lens 5, the following relation holds on the basis ofthe geometric conditions of the optical system shown in FIG. 4:

W1=(B1−2S)×L2/L1+2S  (8)

164. Substitution of this relation into the expression (7) gives:

W1<2×{{square root}{square root over (R²−Q²)}− S}× L2/L1+2S  (9)

165. In the above expressions (5) and (9), R and S represent theaperture radius of the objective lens 5 and the distance between theeffective positions of a virtual light source 200 a supposed to emit thesame beam as a +1st diffracted beam DB1 when no grating surface 3 a isprovided in FIG. 4 and the light source or between those of a virtuallight source 200 b supposed to emit the same beam as a −1st diffractedbeam DB2 and the light source 200, and L1 and L2 represent effectivedistances between the center of the objective lens 5 and the lightsource 200 and between the grating surface 3 a of the diffractiongrating 3 and the light source 200 respectively.

166. The term “effective position” indicates the position of the lightsource emitting the same beam in case of receiving influence by therefractive index of the diffraction grating 3 as a beam in case ofneglecting influence by the refractive index of it. For example, FIG. 5shows the relation between the effective position and the actualposition of the light source 200 (the emissive end of the semiconductorlaser device 2). Assuming that n and d represent the refractive indexand the thickness of the diffraction grating 3 respectively, thedistance D between the effective and actual positions P1 and P2 of thelight source 200 shown in FIG. 5 is obtained as follows:

D=(n−1)×d/n  (10)

167. Further, the following relation holds between the effectivedistance L1 between the center of the objective lens 5 and the lightsource 200 and the physical distance X1 between the center of theobjective lens 5 and the actual position of the light source 200:

L1=X1−D=X1−(n−1)×d/n

168. In addition, the following relation holds between the effectivedistance L2 between the grating surface 3 a of the diffraction grating 3and the effective position of the light source 200 and the physicaldistance X2 between the grating surface 3 a of the diffraction grating 3and the actual position of the light source 200:

L2=X2−D=X2−(n−1)×d/n

169. Assuming that λ and Λ represent the wavelength of the laser beam Band the grating cycle of the diffraction grating of the grating surface3 a respectively, L2 represents the effective distance between theeffective position of the light source 200 and the surface of thediffraction grating 3 closer to the light source 200, and d and nrepresent the thickness of a substrate for the diffraction grating 3 andits refractive index respectively, the distance S between the effectivepositions of the light source 200 and the virtual light sources 200 aand 200 b for the ±1st order diffracted beams in the expressions (5) and(9) is obtained through the following expression (11) or (12) when thegrating surface 3 a is located on the surface of the diffraction grating3 closer to the light source 200 or the objective lens 5:

S={L2+(n−1)×d/n}×tan{sin ⁻¹(λ/Λ)}  (11)

S={L2+d)×tan{sin⁻¹(λ/Λ)}  (12)

170. The grating surface 3 a satisfying the above relation can preventthe ±1st order diffracted beams diffracted and transmitted through thesame from displacement beyond the aperture 5 a of the objective lens 5following movement of the objective lens 5 by Q in the direction X forthe tracking operation.

171. (2) Elliptic Grating Surface 3 a

172. (a) In Relation to an Elliptic Light Spot Coming into Contact withthe Outer Periphery of the Lens Aperture at Two Points on the ObjectiveLens Plane

173.FIG. 6A is a plan view of another diffraction grating 3 and FIG. 6Bis a plan view showing an incident state of diffracted beams, diffractedby the diffraction grating 3 shown in FIG. 6A, on the objective lens 5.

174. Referring to FIG. 6A, a grating surface 3 a of the diffractiongrating 3 is formed in an elliptic shape having a minor axis WA and amajor axis WB, with irregularities extending in the X-axis direction atregular pitches. The grating surface 3 a is formed to be smaller than alight spot (not shown) formed on the diffraction grating surface 3 a bythe laser beam B emitted from the semiconductor laser device 2.

175. Diffracted beams diffracted by the grating surface 3 a in the ±1storder directions form an elliptic light spot BS in the aperture 5 a ofthe objective lens 5, as shown in FIG. 6B. The light spot BS has a minoraxis 2 a and a major axis 2 b corresponding to the minor axis WA and themajor axis WB of the grating surface 3 a respectively.

176. The major axis 2 b of the light spot BS is limited to be smallerthan the diameter of the aperture 5 a of the objective lens 5. Further,the amount Q of movement of the objective lens 5 in the direction X forthe tracking operation is previously defined. Therefore, the minor axisWA of the grating surface 3 a is so set that the light spot BS is notdisplaced beyond the aperture 5 a of the objective lens 5 followingmovement by Q in the X direction for the tracking operation.

177. On the basis of this condition, the minor axis WA of the gratingsurface 3 a is obtained in consideration of the geometric conditions ofthe optical system shown in FIG. 3.

178. Assuming that R and b represent the radius of the aperture 5 a ofthe objective lens 5 and that of the light spot BS along its major axison the objective lens 5 respectively, the minor axis WA of the gratingsurface 3 a is set by the following expression (14), within the rangesatisfying the condition of the following expression (13):

2×[L2/L1×{{square root}{square root over (R×(R−Q))}−S}+S]≦WB<2×[L2/L1×{{square root}{square root over (R²−Q²)}− S}+S]  (13)

WA≦2×{{square root}{square root over (b²Q²/(b²−R²)+b²)}}× L2/L1  (14)

179. where b=(WB−2S)×L1/L2+2S.

180. The variables L1, L2 and S in the expressions (13) and (14) areidentical to those for the rectangular grating surface 3 a.

181. (b) In Relation to an Elliptic Light Spot Coming into Contact withthe Outer Periphery of the Lens Aperture at One Point on the ObjectiveLens Plane

182.FIG. 7A is a plan view of still another diffraction grating 3, andFIG. 7B is a plan view showing an incident state of diffracted beams,diffracted by the diffraction grating 3 shown in FIG. 7A, on theobjective lens 5.

183. Referring to FIG. 7A, a grating surface 3 a of the diffractiongrating 3 is formed in an elliptic shape having a major axis WA and aminor axis WB, with irregularities extending in the direction X atregular pitches. The grating surface 3 a is formed to be smaller than alight spot (not shown) formed on the grating surface 3 a by the laserbeam B emitted from the semiconductor laser device 2.

184. The diffracted beams diffracted by the grating surface 3 a in the±1st order directions form an elliptic light spot BS in the aperture 5 aof the objective lens 5, as shown in FIG. 7B. The light spot BS has amajor axis 2 a and a minor axis 2 b corresponding to the major axis WAand the minor axis WB of the grating surface 3 a respectively.

185. The minor axis 2 b of the light spot BS is limited to be smallerthan the diameter of the aperture 5 a of the objective lens 5. Further,the amount Q of movement of the objective lens 5 in the direction X forthe tracking operation is previously defined. Therefore, the major axisWA of the grating surface 3 a is so set that the light spot BS is notdisplaced beyond the aperture 5 a of the objective lens 5 followingrelative movement by Q in the X direction for the tracking operation.

186. On the basis of this condition, the major axis WA of the gratingsurface 3 a is obtained in consideration of the geometric conditions ofthe optical system shown in FIG. 4.

187. Assuming that R represents the radius of the aperture 5 a of theobjective lens 5, the major axis WA of the grating surface 3 a is set bythe following expression (16), within the range satisfying the conditionof the following expression (15):

WB<2×[L2/L1×{{square root}{square root over (R×(R−Q))}− S}+S ]  (15)

WA≦2×(R−Q)×L2/L1  (16)

188. The variables L1, L2 and S in the expressions (15) and (16) areidentical to those for the rectangular grating surface 3 a.

189. Thus, the grating surface 3 a of the diffraction grating 3 isformed in the elliptic shape along the aforementioned conditions, sothat the ±1st order diffracted beams are located in the aperture 5 a ofthe objective lens 5 following movement of the objective lens 5 for thetracking operation and the subspots S1 and S2 can be prevented fromchange of the light quantities.

190. (c) A Method of Calculating the Dimensions of the Elliptic GratingSurface 3 a

191. The dimensions of the elliptic grating surface 3 a shown in theabove (a) or (b) are calculated in the following manner: Assuming thatR, a and b represent the radius of the aperture 5 a of the objectivelens 5 and those of the light spot BS shown in FIG. 6B in the directionsX and Y respectively, for example, a circle indicating the aperture 5 aof the objective lens 5 is expressed as follows:

X ² +Y ² =R ²  (17)

192. Each ellipse, shown by a dotted line, obtained by moving the lightspot BS in the direction X by the amount Q of movement of the objectivelens 5 is expressed as follows:

(x−Q)² /a ² +Y ² /b ²=1  (18)

193. The intersection between the circle and the ellipse expressed inthe above expressions (17) and (18) is obtained as the condition forpreventing the light spot BS from displacement beyond the aperture 5 aof the objective lens 5. In this case, the following expression (19) isobtained from the expressions (17) and (18):

(a ² −b ²)X ²+2b ² QX+a ² b ² −b ² Q ² −a ² R ²=0  (19)

194. In order to bring the circle and the ellipse expressed in theexpressions (17) and (18) into contact with each other as shown in FIG.6B or 7B, the expression (19) must be transformed into:

(CX−K)²=0   (20)

195. Hence, the following relations are deduced:

a ² −b ² =C ²  (21)

2b ² Q=−2CK  (22)

a ² b ² −b ² Q ² −a ² R ² =K ²  (23)

196. The expression (22) results in b⁴Q²=C²K², which is substituted inthe expressions (21) and (23) to obtain:

a ² {a ²(b ² −R ²)−b ² Q ² −b ²(b ² −R ²)}=0   (24)

197. where a>0, to result in:

a ² (b² −R ²)−b² Q ² −b ² (b ² −R ²)=0  (25)

198. Thus, the following relation holds:

a ² =b ² Q ²/(b ² −R ²)+b ²

a={square root}(b ² Q²/(b ² −R ²)+b ²)  (26)

199. where (26)>0, to result in:

b<{square root}{square root over (R²−Q²)}  (27)

200. As to the condition for bringing the aperture 5 a of the objectivelens 5 and the light spot BS into contact with each other on theboundary between the states shown in FIGS. 6B and 7B following movementof the objective lens, the expression (27) is transformed as follows,with the relation a=R−Q:

b ² −R(R−Q)=0  (28)

201. Thus, the following condition is obtained:

b={square root}{square root over (R(R−Q))}  (29)

202. When the light spot BS comes into contact with the aperture 5 a ofthe objective lens 5 in the state shown in FIG. 6B, i.e.,

{square root}{square root over (R(R−Q))}≦ b<{square root}{square rootover (R²−Q²)}  (30)

203. the light spot BS formed on the objective lens 5 is not displacedbeyond the aperture 5 a under the following condition:

a≦{square root}{square root over (b² Q²/(b²−R²)+b²)}  (31)

204. When the light spot BS comes into contact with the aperture 5 a ofthe objective lens 5 in the state shown in FIG. 7B, i.e.,

b<{square root}{square root over (R(R−Q))}  (32)

205. the light spot BS is not displaced beyond the aperture 5 a underthe following condition:

a≦R−Q   (33)

206. Under the conditions of the above expressions (30) to (33) and inconsideration of the geometric conditions of the optical system shown inFIGS. 3 and 4, the dimensions a and b on the objective lens 5 aretransformed into the vertical and transverse dimensions WA and WB of thegrating surface 3 a, to obtain the relations of the above expressions(13) to (16).

207. The grating surface 3 a is not restricted to the aforementionedrectangular or elliptic shape, but can be formed in any shape such as acircle, a combination of a circle and a rectangle, or a combination of asemi-ellipse and a rectangle, for example. Limit conditions similar tothe above can be applied to such a shape, for setting the dimensions.Particularly in case of a circle, the outer dimensions can be obtainedby applying the above expressions (13) to (16).

208. (2) Second Embodiment

209. An optical pickup apparatus 100 shown in FIG. 8 is adapted toperform focus servo control with the astigmatism method and trackingservo control with the three-beam method.

210. Referring to FIG. 8, symbols X, Y and Z denote the radial directionof a reflection type optical disk 1 such as a CD (compact disk), thetrack direction of the optical disk 1 and a direction perpendicular to arecording plane 1 a of the optical disk 1 respectively.

211. The optical pickup apparatus 100 comprises aprojecting/photoreceiving unit 10 and an objective lens 5. Theprojecting/photoreceiving unit 10 is formed by a semiconductor laserdevice 2, a transmission-type diffraction grating 13, atransmission-type holographic optical element 4 and a photodetector 6.

212. A block 8 is provided on a base 7, and a heat sink 9 is mounted ona side surface of the block 8. The semiconductor laser device 2 ismounted on a surface end of the heat sink 9.

213. The diffraction grating 13 is made of optical glass or opticalresin, and arranged in a holder 11 through a spacer 12. Thetransmission-type holographic optical element 4 is arranged in anopening on an upper surface of the holder 11.

214. The semiconductor laser device 2 emits a laser beam in thedirection Z. The diffraction grating 13 divides the beam emitted fromthe semiconductor laser device 2 into three beams, i.e., a 0th orderdiffracted beam (main beam), a +1st order diffracted beam (subbeam) anda −1st order diffracted beam (subbeam) in a plane substantiallyincluding the directions Y and Z, and transmits the same through thetransmission-type holographic optical element 4. FIG. 8 shows the threebeams as a single beam.

215. The objective lens 5 is supported to be movable in the radialdirection (the direction X) of the optical disk 1 for tracking servocontrol, and to be movable in the vertical direction (the direction Z)for focus servo control. The objective lens 5 condenses the main beamand the two subbeams diffracted and transmitted through thetransmission-type holographic optical element 4 in the 0th and ±1storders respectively on the optical disk 1 as a main spot M0 and subspotsS1 and S2 positioned on both sides thereof respectively (see FIG. 22).

216. The transmission-type holographic optical element 4 diffracts threereturned beams (reflected beams) from the optical disk 1 in a planesubstantially including the directions X and Z in the 1st order andintroduces the same into the photodetector 6. At this time, thetransmission-type holographic optical element 4 supplies the threereturned beams from the optical disk 1 with astigmatism respectively.

217. The photodetector 6, which is similar in structure to thephotodetector 106 of the conventional optical pickup apparatus 100 shownin FIG. 23, outputs an information reproduction signal and a focussignal on the basis of the returned beam from the main spot M0 on theoptical disk 1 while outputting a tracking error signal TE on the basisof the returned beams from the subspots S1 and S2.

218.FIG. 9 is a model diagram showing the optical disk 1 irradiated withthe laser beam B. FIG. 10 is a plan view of the diffraction grating 13receiving the laser beam B. A method of setting the width W (the widthin the direction Y) of a grating surface 13 a of the diffraction grating13 is now described.

219. As shown in FIGS. 21 and 24A, the diffraction grating 103 of theconventional optical pickup apparatus 100 is provided with the gratingsurface 103 a which is larger in width than the laser beam B, having anelliptic sectional shape, along the major axis direction. When theoptical axis LP of the laser beam B deviates from the central axis Z0 ofthe objective lens 105, therefore, the light quantities of the twosubspots S1 and S2 formed on the optical disk 1 are dispersed todisadvantageously offset the tracking error signal TE.

220. In the diffraction grating 13 according to this embodiment, thewidth W of the grating surface 13 a is so adjusted as to cause nodispersion of light quantities in the two subspots S1 and S2 condensedon the optical disk 1 even if the optical axis LP of the laser beam Bdeviates from the central axis Z0 in the direction perpendicular to theradial direction of the optical disk 1.

221. Referring to FIG. 9, the laser beam B emitted from the a lightsource 200 (the semiconductor laser device 2) includes a partial beamBE0 which is common to a +1st order diffracted beam DB1 and a −1st orderdiffracted beam DB2 diffracted by the grating surface 13 a to enter theobjective lens 5, and the width W of the grating surface 13 a is set tobe equal to or smaller than the width A1 of a light spot formed by thecommon partial beam BE0 on the diffraction grating 13.

222. The partial beam BE0 is common to the +1st order diffracted beamDB1 and the −1st order diffracted beam DB2. Even if the optical axis LPof the laser beam B deviates to move the light intensity peak positionas shown in FIG. 26B, therefore, the +1st diffracted beam DB1 and the−1st diffracted beam DB2 change in the same light intensity. Thus, thelight quantities of the two subspots S1 and S2 condensed on the opticaldisk 1 also equally change. Consequently, offset of the tracking errorsignal TE can be prevented.

223. Assuming that R represents the aperture radius of the objectivelens 5, S represents the distance between the effective positions of avirtual light source 200 a for the +1st order diffracted beam DB1 andthe light source 200, and L1 and L2 represent effective distancesbetween the center of the objective lens 5 and the effective position ofthe light source 200 and between the diffraction surface of thediffraction grating 13 and the effective position of the light source200 respectively in FIG. 9, the width A1 of a light spot formed by thepartial beam BE0 incident on the diffraction grating 13 is obtainedthrough the following expression (41):

A1=2×{(R+S)×L2/L1−S}  (41):

224. Hence, the width W of the grating surface 13 a of the diffractiongrating 13 is set to satisfy:

W≦2×{(R+S)×L2/L1−S}

225. The effective position of the light source 200 is that in case ofneglecting influence by the refractive index of the diffraction grating13, and FIG. 11 shows the relation between the effective position andthe actual position of the light source 200 (the emissive end of thesemiconductor laser device 2). Assuming that n and d represent therefractive index and the thickness of the diffraction grating 13 in FIG.11 respectively, the distance D between the effective position P1 andthe actual position P2 of the light source 200 is obtained as follows:

D=(n−1)×d/n  (42)

226. Hence, the following relation holds between the effective distanceL1 between the center of the objective lens 5 and the light source 200and the physical distance X1 between the center of the objective lens 5and the actual position P2 of the light source 200:

L1=X1−D=X1−(n−1)×d/n

227. Further, the following relation holds between the effectivedistance L2 between the grating surface 13 a of the diffraction grating13 and the effective position P1 of the light source 200 and thephysical distance X2 between the grating surface 13 a of the diffractiongrating 13 and the actual position P2 of the light source 200:

L2=X2−D=X2−(n−1)×d/n

228. The virtual light sources 200 a and 200 b are supposed to emit thesame beams as the ±1st order diffracted beams DB1 and DB2 respectivelywhen no diffraction grating 13 is provided.

229. The distance S between the effective position of the light source200 and those of the virtual light sources 200 a and 200 b for the ±1storder diffracted beams DB1 and DB2 in the expression (41) is obtained inthe following manner: Assuming that λ and Λ represent the wavelength ofthe laser beam B and the grating cycle of the grating surface 13 a ofthe diffraction grating 13 respectively, L2 represents the effectivedistance between the effective position of the light source 200 and asurface of the diffraction grating 13 closer to the light source 200 andd and n represent the thickness and the refractive index of a substratefor the diffraction grating 13 respectively, the distance S is obtainedin the following expression (43) or (44) when the grating surface 13 ais provided on a surface of the diffraction grating 13 closer to thelight source 200 or the objective lens 5:

S={L2+(n−1)×d/n}×tan{sin⁻¹ (λ/Λ)}  (43)

S=(L2+d)×tan{sin⁻¹ (λ/Λ)}  (44)

230. Referring to FIG. 9, the width W of the grating surface 13 a of thediffraction grating 13 may be smaller than the width A2 of a light spotformed by the laser beam B on an incidence plane on the diffractiongrating 13. The width A2 of the light spot, including a light spot(first light spot) on the diffraction grating 13 corresponding to a partof the +1st order diffracted beam DB1, diffracted by the diffractiongrating 31, entering the objective lens 5 and a light spot (second lightspot) on the diffraction grating 13 corresponding to a part of the −1storder diffracted beam DB2, diffracted by the diffraction grating 13,entering the objective lens 5, is referred to as an effective beamdiameter.

231. In this case, a partial beam BE5 contributing to the +1st orderdiffracted beam DB1 and a partial beam BE6 contributing only to the −1storder diffracted beam DB2 further enter the grating surface 13 a in thelaser beam B. However, the width W of the grating surface 13 a isrendered smaller than the width A2 of the light spot formed by the laserbeam B, thereby reducing the incident quantities of the partial beamsBE5 and BE6. Therefore, the ratio of the partial beams BE5 and BE6changing the light quantities due to optical axis deviation of the laserbeam B reduces with respect to the partial beam BE0 not influenced bythe optical axis deviation. Thus, the quantity of offset of the trackingerror signal TE resulting from optical axis deviation can be reduced ascompared with the conventional diffraction grating 103.

232. The width A2 of the light spot formed by the laser beam B isobtained as follows:

A2=2×{(R−S)×L2/L1+S}  (45)

233. In this case, therefore, the width W of the grating surface 13 a isset to satisfy the following relation:

W<A2

234. The variables in the expression (45) are similar to those in theabove expressions (41) to (44).

235. The diffraction grating 13 of this embodiment is applicable notonly to the optical pickup apparatus 100 shown in FIG. 8 verticallyemitting the laser beam B, but also to an optical pickup apparatushorizontally emitting a laser beam and vertically guiding the same witha reflecting mirror.

236. In the aforementioned second embodiment, the limit value for thewidth W of the grating surface 13 a of the diffraction grating 13 isobtained for suppressing offset of the tracking error signal TEresulting from deviation of the optical axis of the laser beam B and thecentral axis of the objective lens 5 in the direction perpendicular tothe radial direction of the optical disk 1, i.e., in the direction Y.Thus, the dimensions of the grating surface 13 a obtained in the firstembodiment can be further set while satisfying the limit condition forthe width W thereof. When the grating surface 3 a (13 a) has arectangular shape, for example, the dimension of the grating surface 3 a(13 a) in the direction (the direction Y) perpendicular to the radialdirection of the optical disk 1 is set to satisfy the limitation of theexpression (45), preferably the expression (41) in the secondembodiment, and the limitation of the expression (9) in the firstembodiment, and that in the radial direction (the direction X) of theoptical disk 1 is set to satisfy the limitation of the expression (5) inthe first embodiment. Thus, it is possible to suppress offset of thetracking error signal TE resulting from deviation of the optical axis ofthe laser beam B and the central axis of the objective lens 5, whilepreventing output reduction of the tracking error signal TE in thetracking operation. A similar effect can be attained also in relation toan elliptic or circular grating surface.

237. (3) Third Embodiment

238.FIG. 12 is a side elevational view showing the structure of anoptical pickup apparatus according to a third embodiment of the presentinvention, and FIG. 13 is an exploded perspective view of aprojecting/photoreceiving unit provided in the optical pickup apparatusshown in FIG. 12. Referring to FIGS. 12 and 13, symbols X, Y and Zdenote the radial direction of an optical disk 1, a directionperpendicular to a recording plane of the optical disk 1, and adirection perpendicular to the X-Y plane. FIG. 12 shows theprojecting/photoreceiving unit in a state rotated by 90° about the Yaxis, for the convenience of illustration.

239. Referring to FIGS. 12 and 13, the optical pickup apparatus isformed by integrally assembling the projecting/photoreceiving unitincluding a semiconductor laser device 21, a diffraction grating 23 anda transmission-type holographic optical element 24 into a housing (notshown) including an objective lens 16.

240. The projecting/photoreceiving unit comprises a support member 30.The support member 30 is formed by integrating a lead frame 32 with apair of leads 33 a and 33 b by an insulating molded body 31 which ismade of resin. The insulating molded body 31 is provided on its uppersurface with a concave part 34 opening between first and second endsurfaces 35 and 36 for exposing surfaces of the lead frame 32 and theleads 33 a and 33 b.

241. A conductive submount (heat sink) 20 is mounted on the surface ofthe lead frame 32 exposed in the concave part 34 of the insulatingmolded body 31 at a portion closer to the first end surface 35, to beelectrically connected with the lead frame 32. A monitor photodiode 22is formed on a part of the upper surface of the conductive submount 20.The semiconductor laser device 21 is mounted on the upper surface of thesubmount 20 in front of the monitor photodiode 22. The semiconductorlaser device 21 emits laser beams from its front and rear end surfacesrespectively, so that the monitor photodiode 22 receives the laser beamemitted from the rear end surface as a monitor beam.

242. The diffraction grating 23 is arranged in the concave part 34 ofthe insulating molded body 31 on the central portion of the lead frame32. A grating surface of the diffraction grating 32 divides the laserbeam emitted from the front end surface of the semiconductor laserdevice 21 into 0th, +1st and −1st order diffracted beams.

243. In the concave part 34 of the insulating molded body 31, thetransmission-type holographic optical element 24 is arranged on thesurface of the lead frame 32 in a portion closer to the second endsurface 36. A holographic surface 25 of the transmission-typeholographic optical element 24 transmits the 0th, +1st and −1st orderdiffracted beams from the diffraction grating 23, and diffracts returnedbeams from an optical recording medium 1 such as an optical disk. In theconcave part 34 of the insulating molded body 31, further, a stray lightscreen 37 is arranged on the lead frame 32 for shielding the beamemitted from the semiconductor laser device 21 and the returned beamsfrom the diffraction grating 23 against each other.

244. A flexible circuit board 50 is mounted on the first end surface 35of the insulating molded body 31. The flexible circuit board 50, whichis formed by a polyimide resin plate provided with a conductive wiringpattern on its surface, has a wiring part 51 and fixing parts 52. Aphotodiode integrated circuit device (hereinafter referred to as a PDIC)45 for signal detection serving as a photodetector is mounted on asurface (provided with the wiring pattern) of the wiring part 51 of theflexible circuit board 50. Circular and elliptic screw through holes 54a and 55 b are formed in the wiring part 51 of the flexible circuitboard 50 on both sides of the PDIC 45. Further, a keep plate 56 ismounted on the rear surface of the flexible circuit board 50.

245. Surfaces of the fixing parts 52 of the flexible circuit board 50are fixed to lower surfaces of the pair of leads 33 a and 33 b and thelead frame 32 by soldering or the like. The wiring part 51 of theflexible circuit board 50 is bent upward, to be perpendicular to theupper surface of the lead frame 32. In this state, screws 57 and 58 arefitted into screw holes (not shown) through the screw through holes 54 aand 55 b, for mounting the wiring part 51 on the first end surface 35 ofthe insulating molded body 31.

246. A reflecting mirror 15, an objective lens 16 and an actuator 60 aremounted on the housing. The reflecting mirror 15 vertically reflects thethree diffracted beams transmitted through the transmission-typeholographic optical element 24 upward, while horizontally reflecting thereturned beams from the optical disk 1 and guiding the same to thetransmission-type holographic optical element 24.

247. The objective lens 16 condenses the three diffracted beamsreflected by the reflecting mirror 15 on the optical disk 1 for forminga main spot and two subspots positioned on both sides thereof.

248. The actuator 60 has a holder 61, a tracking coil 62, a yoke 63 andpermanent magnets 64. When supplied with a driving signal (trackingerror signal), the tracking coil 62 receives electromagnetic forcecaused between the permanent magnets 64 mounted on the fixed yoke 63 andthe tracking coil 62 for moving the objective lens 16 in the radialdirection (the X-axis direction) of the optical disk 1 through theholder 61.

249.FIG. 14 is a plan view of the flexible circuit board 50 employed inthe optical pickup apparatus shown in FIG. 12, FIG. 15A illustratesexemplary wiring on the flexible circuit board 50 shown in FIG. 14, andFIG. 15B is a plan view of photoreceiving parts of the PDIC 45 arrangedon the flexible circuit board 50 shown in FIG. 14.

250. As shown in FIG. 14, the flexible circuit board 50 is formed by apolyimide resin plate 65 provided with a plurality of conductive wiringlayers L1 to L10. As shown in FIGS. 14, 15A and 15B, the wiring layersL1 to L6 are connected to output electrodes of the signal detection PDIC45 through bonding wires, and the wiring layer L7 is connected to a GNDelectrode of the signal detection PDIC 45. The wiring layer L8 isconnected to an anode of the monitor photodiode 22 through the lead 33 band a bonding wire, the wiring layer L9 is connected to an anode of thesemiconductor laser device 22 through the lead 33 a and a bonding wire19, and the wiring layer L10 is connected to cathodes of thesemiconductor laser device 21 and the monitor photodiode 22 in commonthrough the lead frame 32.

251. The circular and elliptic screw through holes 54 a and 55 b areformed on both sides of the signal detection PDIC 45. Referring to FIGS.15A and 15B, the signal detection PDIC 45 includes photodetection parts70 a to 70 d provided on the central portion for performing focus servocontrol with the astigmatism method, photodetection parts 70 e and 70 fprovided on both sides of the photodetection parts 70 a to 70 d forperforming tracking servo control with the three-beam method, andoperational amplifiers 72 a to 72 d. Detection signals of thephotodetection parts 70 a to 70 d are outputted through the operationalamplifiers 72 a to 72 d and four of the wiring layers L1 to L6, whilethose of the photodetection parts 70 e and 70 f for tracking servocontrol are outputted through the operational amplifiers 72 e and 72 fand the remaining two of the wiring layers L1 to L6. An adjustingcircuit 71 (see FIG. 16) described later is connected to the wiringlayers connected to the photodetection parts 70 e and 70 f.

252.FIG. 16 shows the circuit structures of respective parts of theaforementioned optical pickup apparatus 100 for performing a trackingoperation. The optical pickup apparatus 100 performs the trackingoperation through an inspection driving circuit 74 in an inspectionstep. As shown in FIG. 16, the optical pickup apparatus 100 is providedwith the adjusting circuit 71 in an intermediate stage of the wiringlayers outputting the detection signals of the photodetection parts 70 eand 70 f of the signal detection PDIC 45. The adjusting circuit 71 hasresistances R1 and R2 serially inserted in the wiring layers connectedto the photodetection parts 70 e and 70 f and a variable resistor VRinserted between the wiring layers. A movable terminal of the variableresistor VR is connected to a power supply voltage Vcc. The resistancevalue of the variable resistor VR is changed thereby changing detectionsignals E0 and F0 outputted from the photodetection parts 70 e and 70 fof the signal detection PDIC 45 respectively. Thus, it is possible togenerate and output a desired detection signal for correcting opticalaxis deviation of the objective lens 16 described later.

253. In this embodiment, the semiconductor laser device 21 correspondsto the light source, the diffraction grating 23 corresponds to thediffraction element, the signal detection PDIC 45 corresponds to thephotodetector, and the adjusting circuit 71 corresponds to an adjustingpart and the adjusting circuit.

254. In the optical pickup apparatus having the aforementionedstructure, the housing provided with the objective lens 16 and theprojecting/photoreceiving unit provided with the semiconductor laserdevice 21 and the like are integrally assembled with each other inalignment. In the assembling step for the optical pickup apparatus, themounting position of the objective lens 16 with respect to the opticalaxis of the laser beam B emitted from the semiconductor laser device 21may deviate in the radial direction (the X-axis direction) of theoptical disk 1 due to an assembling error.

255. Therefore, the aforementioned optical pickup apparatus 100 isprovided with the inspection driving circuit 74 as shown in FIG. 16, foradjusting the resistance value of the variable resistor VR of theadjusting circuit 71 provided on the optical pickup apparatus 100 andcorrecting the deviation of the objective lens 16.

256. First, the optical pickup apparatus 100 is mounted on a prescribedposition of an inspection apparatus (not shown), and the semiconductorlaser device 21 irradiates the optical disk 1 with the laser beam, forforming the main spot for reproduction and the two subspots for trackingstate detection. The signal detection PDIC 45 receives the returnedbeams corresponding to the main spot and the subspots respectively, andoutputs the detection signals E0 and F0 corresponding to the receivedlight quantities. The detection signals E0 and F0 pass through theadjusting circuit 71, to be outputted from output terminals of theflexible circuit board 50 to the inspection driving circuit 74 asdetection signals E and F.

257. The inspection driving circuit 74 has an E-F processing part 75, alow-pass filter 76 and an operational amplifier 77. The E-F processingpart 75 calculates a tracking error signal TE (=E−F) on the basis of thedetection signals E and F outputted from the optical pickup apparatus100. In this inspection step, the tracking error signal TE is employednot for actually performing tracking but for supplying the tracking coil62 with a bias voltage for forcibly moving the deviating objective lens16 along the radial direction of the optical disk 1. This tracking errorsignal TE passes through the low-pass filter 76 and is amplified by theoperational amplifier 77, to be supplied to the tracking coil 62. Thus,the objective lens 16 is moved along the radial direction of the opticaldisk 1 in response to the tracking error signal TE.

258. Adjustment of the adjusting circuit 71 is performed as follows: Theinspector supplies a constant driving signal to the tracking coil 62 formoving the objective lens 16 along the radial direction of the opticaldisk 1 toward the center and the outer periphery respectively by aconstant distance of 400 μm, for example, and compares the voltages ofthe tracking error signal TE with each other. If the tracking errorsignal TE exhibits different values following movement in the oppositedirections as shown in FIG. 32, the inspector adjusts the resistancevalue of the variable resistor VR of the adjusting circuit 71 forchanging the voltages of the detection signals E0 and F0 and equalizingthe values of the tracking error signal TE with each other. Thus, theadjusting circuit 71 adds a constant bias voltage to the driving signalapplied to the tracking coil 62, for correcting the deviation of theobjective lens 16.

259.FIG. 17 is a circuit diagram of an adjusting circuit 73 according toa modification of this embodiment. In the optical pickup apparatus shownin FIG. 17, operational amplifiers 72 e and 72 f for amplification areprovided on output sides of photodetection parts 70 e and 70 frespectively. A reference voltage Vref is inputted in a first input partof the operational amplifier 72 e provided for the photodetection part70 e. This operational amplifier 72 e amplifies the difference between adetection signal from the photodetection part 70 e and the referencevoltage Vref, and outputs the same as a detection signal E.

260. The adjusting circuit 73 is connected to a first input side of theoperational amplifier 72 f. The adjusting circuit 73 is connected to apower supply voltage Vcc, to be capable of changing the voltage of areference signal inputted in the operational amplifier 72 f by adjustinga variable resistor 73 a. The operational amplifier 72 f amplifies thedifference between a detection signal from the photodetection part 70 fand the reference signal from the adjusting circuit 73 and outputs thesame as a detection signal F. The adjusting circuit 73 can change thevalue of the detection signal F. Thus, a bias voltage for correctingdeviation of the objective lens 16 can be added to the tracking errorsignal TE calculated by the E-F processing part 75. Consequently, theobjective lens 16 is moved in the radial direction by the driving signalsupplied to the tracking coil 62, for correcting deviation of theobjective lens 16 and the optical axis of the laser beam B in the radialdirection.

261. In this embodiment, the photodetection parts 70 a to 70 f and theoperational amplifiers 72 a to 72 f (amplifier parts) are formed on asingle chip.

262. The operational amplifiers 72 e and 72 f of this embodimentcorrespond to the amplifier part of the present invention.

263.FIG. 18 shows another exemplary signal detection PDIC 45. Aphotoreceiving part 85 of this signal detection PDIC 45 comprises a pairof photodetection parts 86 a and 86 b for focus servo control and a pairof photodetection parts 86 c and 86 d for tracking servo control whichare arranged to be opposed to the photodetection parts 86 a and 86 b. Inresponse to this photodetection parts of the signal detection PDIC 45, aholographic surface of the transmission-type holographic optical element24 is divided into four regions having different shapes. The adjustingcircuit 71 or 73 according to the third embodiment or the modificationthereof can also be provided on the signal detection PDIC 45 having suchphotodetection parts 86 a to 86 d.

264. Thus, the optical pickup apparatus according to this embodimentcomprises the adjusting circuit 71 or 73 and is capable of correctingdeviation of the objective lens 16 by itself, whereby a manufacturer foran apparatus to be assembled with the optical pickup apparatus requiresno operation for adjusting deviation of the objective lens 16.

265. (4) Fourth Embodiment

266.FIG. 19 is a block diagram showing the structure of an opticalrecording medium drive 90 employing the optical pickup apparatus 100according to any of the first to third embodiments of the presentinvention. The optical recording medium drive 90 shown in FIG. 19 is anoptical disk drive for reading information from an optical disk 1. Theoptical recording medium drive 90 includes the optical pickup apparatus100, a motor 91, a feed motor 92, a rotation control system 93, a signalprocessing system 94, a pickup control system 95, a feed motor controlsystem 96 and a drive controller 97.

267. The motor 91 rotates the optical disk 1 at a prescribed speed. Therotation control system 93 controls the rotating operation of the motor91. The feed motor 92 moves the optical pickup apparatus 100 in theradial direction of the optical disk 1. The feed motor control system 96controls the operation of the feed motor 92. The optical pickupapparatus 100 irradiates the optical disk 1 with a laser beam andreceives a returned beam from the optical disk 1. The pickup controlsystem 95 controls the projecting/photoreceiving operation of theoptical pickup apparatus 100. The signal processing system 94 receives adetection signal from the signal detection PDIC 45 of the optical pickupapparatus 100 and calculates a reproduction signal, a focus error signaland a tracking error signal, for supplying the reproduction signal tothe drive controller 97 while supplying the focus error signal and thetracking error signal to the pickup control system 95. The drivecontroller 97 controls the rotation control system 93, the signalprocessing system 94, the pickup control system 95 and the feed motorcontrol system 96 in accordance with an instruction supplied through thedrive interface 98, and outputs the reproduction signal through thedrive interface 98. According to this embodiment, the motor 91 and therotation control system 93 corresponds to the pickup driving part, andthe signal processing system 94 corresponds to the signal processingpart.

268. When the optical recording medium drive 90 shown in FIG. 19 employsthe optical pickup apparatus 100 according to the first embodiment, itis possible to suppress output fluctuation of the tracking error signalresulting from movement of the objective lens in the tracking operation,for performing the tracking operation in high accuracy.

269. When employing the optical pickup apparatus 100 according to thesecond embodiment, it is possible to perform the tracking operation inhigh accuracy while suppressing offset of the tracking error signalresulting from optical axis deviation of the laser beam. When employingthe optical pickup apparatus 100 according to the third embodiment,deviation of the objective lens may not be adjusted and the assemblingoperation is simplified.

270. Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

What is claimed is:
 1. An optical pickup apparatus comprising: a lightsource for emitting a beam; a first diffraction element for diffractingsaid beam emitted from said light source at least in first and seconddirections; and an objective lens for irradiating an optical recordingmedium with beams diffracted by said first diffraction element in saidfirst and second directions, said objective lens being provided to bemovable along the radial direction of said optical recording medium fora tracking operation, a diffraction surface of said first diffractionelement being formed in dimensions for locating a light spot beingformed on said objective lens by said beams diffracted by saiddiffraction surface in said first and second directions in an apertureof said objective lens following movement of said objective lens forsaid tracking operation.
 2. The optical pickup apparatus in accordancewith claim 1 , wherein said diffraction surface of said firstdiffraction element is formed in a rectangular shape being smaller thana light spot formed on said first diffraction element by said beamemitted from said light source in dimensions for locating a rectangularspot formed on said objective lens by said diffracted beams diffractedby said diffraction surface in said first and second directionsfollowing movement of said objective lens.
 3. The optical pickupapparatus in accordance with claim 2 , wherein the width W1 of saiddiffraction surface of said first diffraction element in a directionperpendicular to the direction of movement of said objective lens is setto satisfy: W<2×{{square root}{square root over (R²−Q²)}− S}×L2/L1+2Sand the width W2 of said diffraction surface in the direction ofmovement of said objective lens is set to satisfy: W2≦{{squareroot}{square root over ((2R)²−(B1)²)}−2Q}×L2/L1 whereB1=(W1−2S)×L1/L2+2S assuming that R and Q represent the aperture radiusand the distance of movement of said objective lens respectively, L1 andL2 represent effective distances between said light source and thecenter of said objective lens and between said diffraction surface andsaid light source respectively, S represents the distance between afirst virtual light source supposed to emit a straight beam beingequivalent to said beam diffracted in said first direction toward saidobjective lens and said light source or between a second virtual lightsource supposed to emit a straight beam being equivalent to said beamdiffracted in said second direction toward said objective lens and saidlight source, and B1 represents a limit width for said rectangular lightspot formed on said objective lens in said direction perpendicular tothe direction of movement of said objective lens.
 4. The optical pickupapparatus in accordance with claim 1 , further comprising: a seconddiffraction element for transmitting said beams diffracted by said firstdiffraction element in said first and second directions and guiding thesame to said objective lens while diffracting returned beams from saidoptical recording medium, and a photodetector for receiving saidreturned beams diffracted by said second diffraction element.
 5. Theoptical pickup apparatus in accordance with claim 1 , wherein saiddiffraction surface of said first diffraction element is formed in anelliptic or circular shape being smaller than a light spot formed onsaid first diffraction element by said beam emitted from said lightsource in dimensions for locating an elliptic light spot formed on saidobjective lens by said diffracted beams diffracted by said diffractionsurface in said first and second directions following movement of saidobjective lens.
 6. The optical pickup apparatus in accordance with claim5 , wherein said elliptic diffraction surface of said first diffractionelement has a major axis in said direction perpendicular to thedirection of movement of said objective lens, and the width WA of saidelliptic diffraction surface in the direction of movement of saidobjective lens is set to satisfy: WA≦2×{{square root}{square root over(b² Q²/(b²−R²)+b²)}}× L2/L1 where b=(WB−2S)×L1/L2+2S assuming that R andQ represent the aperture radius and the amount of movement of saidobjective lens respectively, L1 and L2 represent effective distancesbetween said light source and the center of said objective lens andbetween said diffraction surface and said light source respectively, Srepresents the distance between a first virtual light source supposed toemit a straight beam being equivalent to said beam diffracted in saidfirst direction toward said objective lens and said light source orbetween a second virtual light source supposed to emit a straight beambeing equivalent to said beam diffracted in said second direction towardsaid objective lens and said light source, b represents a limit widthfor the radius of said elliptic light spot formed on said objective lensin said direction perpendicular to the direction of movement of saidobjective lens and WB represents the width of said elliptic diffractionsurface in said direction perpendicular to the direction of movement ofsaid objective lens.
 7. The optical pickup apparatus in accordance withclaim 6 , wherein said elliptic diffraction surface of said firstdiffraction element is so set that the width WB in said directionperpendicular to the direction of movement of said objective lenssatisfies: 2×[L2/L1×{{square root}{square root over (R×(R−Q))}−S}+S]≦WB<2×[L2/L1× {{square root}{square root over (R²−Q²)}− S}+S] 8.The optical pickup apparatus in accordance with claim 5 , wherein thewidth WA of said elliptic diffraction surface in the direction ofmovement of said objective lens is set to satisfy: WA≦2×(R−Q)×L2/L1assuming that R and Q represent the aperture radius and the amount ofmovement of said objective lens respectively and L1 and L2 representeffective distances between said light source and the center of saidobjective lens and between said diffraction surface and said lightsource respectively.
 9. The optical pickup apparatus in accordance withclaim 8 , wherein said elliptic diffraction surface of said firstdiffraction element is so set that the width WB in said directionperpendicular to the direction of movement of said objective lenssatisfies: WB<2×[L2/L1×{{square root}{square root over (R×(R−Q))}− S}+S]assuming that S represents the distance between a first virtual lightsource supposed to emit a straight beam being equivalent to said beamdiffracted in said first direction toward said objective lens and saidlight source or between a second virtual light source supposed to emit astraight beam being equivalent to said beam diffracted in said seconddirection toward said objective lens and said light source.
 10. Theoptical pickup apparatus in accordance with claim 5 , furthercomprising: a second diffraction element for transmitting said beamsdiffracted by said first diffraction element in said first and seconddirections and guiding the same to said objective lens while diffractingreturned beams from said optical recording medium, and a photodetectorfor receiving said returned beams diffracted by said second diffractionelement.
 11. An optical pickup apparatus comprising: a light source foremitting a beam; a first diffraction element having a diffractionsurface for diffracting said beam emitted from said light source atleast in first and second directions; and an objective lens forirradiating an optical recording medium with beams diffracted by saidfirst diffraction element in said first and second directions, whereinthe width of said diffraction surface of said first diffraction elementin a plane including the optical axis of said beam emitted from saidlight source and axes of said beams diffracted in said first and seconddirections is set to be smaller than the width of a region including afirst light spot and a second light spot, said first light spot being alight spot on said first diffraction element corresponding to a part ofsaid beam, diffracted by said first diffraction element in said firstdirection, entering said objective lens in said beam emitted from saidlight source, and said second light spot being a light spot on saidfirst diffraction element corresponding to a part of said beam,diffracted by said first diffraction element in said second direction,entering said objective lens in said beam emitted from said lightsource.
 12. The optical pickup apparatus in accordance with claim 11 ,wherein the width of said diffraction surface of said first diffractionelement in said plane is set to be smaller than the width of an overlapregion of said first and second light spots on said first diffractionelement.
 13. The optical pickup apparatus in accordance with claim 12 ,wherein said first direction is a +1st order diffraction direction, andsaid second direction is a −1st order diffraction direction.
 14. Theoptical pickup apparatus in accordance with claim 12 , wherein the widthW of said diffraction surface of said first diffraction element is setto satisfy the following relation: W≦2×{(R+S)×L2/L1−S} assuming that Rrepresents the aperture radius of said objective lens, L1 and L2represent effective distances between said light source and the centerof said objective lens and between said diffraction surface and saidlight source respectively and S represents the distance between a firstvirtual light source supposed to emit a straight beam being equivalentto said beam diffracted in said first direction toward said objectivelens and said light source or between a second virtual light sourcesupposed to emit a straight beam being equivalent to said beamdiffracted in said second direction toward said objective lens and saidlight source.
 15. The optical pickup apparatus in accordance with claim14 , wherein said effective distance L1 is defined by: L1=X1−(n−1)×d/nassuming that X1 represents the physical distance between said lightsource and the center of said objective lens and d and n represent thethickness and the refractive index of said first diffraction elementrespectively, and said effective distance L2 is defined by:L2=X2−(n−1)×d/n assuming that X2 represents the physical distancebetween said light source and said diffraction surface and d and nrepresent the thickness and the refractive index of said firstdiffraction element respectively.
 16. The optical pickup apparatus inaccordance with claim 12 , further comprising: a second diffractionelement for transmitting said beams diffracted by said first diffractionelement in said first and second directions and guiding the same to saidobjective lens while diffracting returned beams from said opticalrecording medium, and a photodetector for receiving said returned beamsdiffracted by said second diffraction element.
 17. An optical pickupapparatus comprising: a light source for emitting a beam; a firstdiffraction element having a diffraction surface for diffracting saidbeam emitted from said light source at least in first and seconddirections; and an objective lens for irradiating an optical recordingmedium with beams diffracted by said first diffraction element in saidfirst and second directions, wherein said objective lens is provided tobe movable along the radial direction of said optical recording mediumfor a tracking operation, said diffraction surface of said firstdiffraction element is so formed that the width in a plane including theoptical axis of said beam emitted from said light source and axes ofsaid beams diffracted in said first and second directions is smallerthan the width of a region including a first light spot and a secondlight spot, and dimensions are set for locating a light spot formed onsaid objective lens by said beams diffracted by said diffraction surfacein said first and second directions in an aperture of said objectivelens following movement of said objective lens for said trackingoperation, said first light spot being a light spot on said firstdiffraction element corresponding to a part of said beam, diffracted bysaid first diffraction element in said first direction, entering saidobjective lens in said beam emitted from said light source, and saidsecond light spot being a light spot on said first diffraction elementcorresponding to a part of said beam, diffracted by said firstdiffraction element in said second direction, entering said objectivelens in said beam emitted from light source.
 18. The optical pickupapparatus in accordance with claim 17 , wherein the width of saiddiffraction surface of said diffraction grating in said plane is set tobe smaller than the width of an overlap region of said first and secondlight spots on said first diffraction element.
 19. The optical pickupapparatus in accordance with claim 18 , further comprising: a seconddiffraction element for transmitting said beams diffracted by said firstdiffraction element in said first and second directions and guiding thesame to said objective lens while diffracting returned beams from saidoptical recording medium, and a photodetector for receiving saidreturned beams diffracted by said second diffraction element.
 20. Anoptical pickup apparatus capable of detecting a tracking state of a beamfor reading information from an optical recording medium, said opticalpickup apparatus comprising: a light source for emitting a beam; a firstdiffraction element for dividing said beam emitted from said lightsource into a plurality of beams for tracking state detection; anobjective lens being provided to be movable in the radial direction ofsaid optical recording medium for condensing said plurality of beamsdivided by said first diffraction element on said optical recordingmedium; a photodetector having a plurality of photoreceiving parts forreceiving a plurality of returned beams based on said plurality of beamsfor tracking state detection condensed on said optical recording mediumrespectively and outputting detection signals responsive to receivedlight quantities; an adjusting circuit capable of changing a pluralityof said detection signals outputted from said plurality ofphotoreceiving parts of said photodetector; and a lens driving part formoving said objective lens in said radial direction in response to aprescribed signal being based on said plurality of detection signalsadjusted by said adjusting circuit.
 21. The optical pickup apparatus inaccordance with claim 20 , wherein said adjusting circuit includes avariable resistor for changing said plurality of detection signalsoutputted from said plurality of photoreceiving parts of saidphotodetector.
 22. The optical pickup apparatus in accordance with claim21 , further comprising a wiring part for extracting said signals fromsaid plurality of photoreceiving parts of said photodetector, saidvariable resistor being arranged on said wiring part.
 23. The opticalpickup apparatus in accordance with claim 22 , wherein said wiring partis formed on a flexible circuit board.
 24. The optical pickup apparatusin accordance with claim 20 , further comprising a plurality ofamplifier parts being provided in correspondence to said plurality ofphotoreceiving parts in said photodetector for amplifying thedifferences between said detection signals outputted from correspondingsaid photoreceiving parts and a reference signal respectively, saidadjusting circuit including a variable resistor for changing saidreference signal being supplied to at least one of said plurality ofamplifier parts.
 25. The optical pickup apparatus in accordance withclaim 24 , wherein said photoreceiving parts and said plurality ofamplifier parts are formed on a single chip.
 26. The optical pickupapparatus in accordance with claim 20 , further comprising a seconddiffraction element for transmitting said plurality of beams divided bysaid first diffraction element and guiding the same to said objectivelens while diffracting said plurality of returned beams from saidoptical recording medium and guiding the same to said photodetector. 27.An optical recording medium drive for optically reading information froman optical recording medium, said optical recording medium drivecomprising: a rotation driving part for rotating said optical recordingmedium; an optical pickup apparatus for emitting a laser beam to saidoptical recording medium and receiving a returned beam from said opticalrecording medium; a pickup driving part for moving said optical pickupapparatus in the radial direction of said optical recording medium; anda signal processing part for processing an output signal from saidoptical pickup apparatus, said optical pickup apparatus comprising: alight source for emitting a beam, a diffraction element having adiffraction surface for diffracting said beam emitted from said lightsource at least in first and second directions, and an objective lensfor irradiating said optical recording medium with beams diffracted bysaid first diffraction element in said first and second directions, saidobjective lens being provided to be movable along the radial directionof said optical recording medium for a tracking operation, saiddiffraction surface of said diffraction element being formed indimensions for locating a light spot formed on said objective lens bysaid beams diffracted by said diffraction surface in said first andsecond directions in an aperture of said objective lens followingmovement of said objective lens for said tracking operation.
 28. Anoptical recording medium drive for optically reading information from anoptical recording medium, said optical recording medium drivecomprising: a rotation driving part for rotating said optical recordingmedium; an optical pickup apparatus for emitting a laser beam to saidoptical recording medium and receiving a returned beam from said opticalrecording medium; a pickup driving part for moving said optical pickupapparatus in the radial direction of said optical recording medium; anda signal processing part for processing an output signal from saidoptical pickup apparatus, said optical pickup apparatus comprising: alight source for emitting a beam, a diffraction element having adiffraction surface for diffracting said beam emitted from said lightsource at least in first and second directions, and an objective lensfor irradiating said optical recording medium with beams diffracted bysaid first diffraction element in said first and second directions,wherein the width of said diffraction surface of said diffractionelement in a plane including the optical axis of said beam emitted fromsaid light source and axes of said beams diffracted in said first andsecond directions is set to be smaller than the width of a regionincluding a first light spot and a second light spot, said first lightspot being a light spot on said first diffraction element correspondingto a part of said beam, diffracted by said first diffraction element insaid first direction, entering said objective lens in said beam emittedfrom said light source, and said second light spot being a light spot onsaid first diffraction element corresponding to a part of said firstdiffraction element corresponding to a part of said beam, diffracted bysaid first diffraction element in said second direction, entering saidobjective lens in said beam emitted from said light source.
 29. Anoptical recording medium drive for optically reading information from anoptical recording medium, said optical recording medium drivecomprising: a rotation driving part for rotating said optical recordingmedium; an optical pickup apparatus for emitting a laser beam to saidoptical recording medium and receiving a returned beam from said opticalrecording medium; a pickup driving part for moving said optical pickupapparatus in the radial direction of said optical recording medium; anda signal processing part for processing an output signal from saidoptical pickup apparatus, said optical pickup apparatus comprising: alight source for emitting a beam, a diffraction element for dividingsaid beam emitted from said light source into a plurality of beams fortracking state detection, an objective lens being provided to be movablein the radial direction of said optical recording medium for condensingsaid plurality of beams being divided by said first diffraction elementon said optical recording medium, a photodetector having a plurality ofphotoreceiving parts for receiving a plurality of returned beams basedon said plurality of beams for tracking state detection condensed onsaid optical recording medium respectively and outputting detectionsignals responsive to received light quantities, an adjusting circuitcapable of changing plurality of said detection signals outputted fromsaid plurality of photoreceiving parts of said photodetector, and a lensdriving part for moving said objective lens in said radial direction inresponse to a prescribed signal being based on said plurality ofdetection signals adjusted by said adjusting circuit.
 30. A method ofadjusting an optical pickup apparatus, comprising a light source foremitting a beam, a diffraction element for dividing said beam emittedfrom said light source into a plurality of beams for tracking statedetection, an objective lens for condensing said plurality of beamsdivided by said diffraction element on an optical recording medium, alens driving part for moving said objective lens in the radial directionof said optical recording medium, and a photodetector having a pluralityof photoreceiving parts for receiving a plurality of returned beamsbased on said plurality of beams for tracking state detection condensedon said optical recording medium respectively and outputting a pluralityof detection signals responsive to received light quantities, saidmethod being adapted to correct deviation of a central portion of saidobjective lens with respect to the optical axes of said plurality ofbeams in the radial direction of said optical recording medium by:providing an adjusting circuit capable of changing said detectionsignals outputted from said plurality of photoreceiving parts in saidoptical pickup apparatus, connecting a driving circuit for generating adriving signal for moving said objective lens in said radial directionon the basis of said detection signals outputted from said photodetectorthrough said adjusting circuit to said lens driving part of said opticalpickup apparatus, and moving said objective lens in said radialdirection by changing said detection signals with said adjusting circuitand thereafter observing change of said detection signals while radiallymoving said objective lens in said radial direction by a prescribeddistance, thereby correcting deviation of said central portion of saidobjective lens with respect to the optical axes of said plurality ofbeams in the radial direction of said optical recording medium.